Full text of "Plant Diseases"

See other formats



83 ^ CONGRESS, ISi SESSION 
HOUSE DOCUMENT NO. 122 


the yearbook of agriculture 

1953 




Plant 

Diseases 




1953 


UNITED STATES 
DEPARTMENT OF 
AGRICULTURE 
WASHINGTON, D. C. 



OXFORD & IBH PUBLISHING CO. 

Calcutta ■ Bombay ■ New Delhi 



First Indian Edition 1960 


This book has been published with the assistance 
of the Joint Indian- American Textbook Programme, 


Published by Oxford A IBH Publishing Co ,, Oxford Building, 
hl-S8 Connaught Circus, New Delhi- ! and printed by 
Mohan Makhijani, Rekha Printers, New Delhi-55 



tk* yearbook 
committee 

Bureau of Plant Industry^ SoilSy and 
Agricultural Engineering 

CURTIS MAY, chairman 

PHILIP BRtERLEY 

EDWARD E- CLAYFON 
JOHN C. DUNEGAN 
KERMIT VV. KREITLOW 

W. D. Me CLELLAN 

PAUL R. MILI^ER 

H. A. RODENHISER 
W. j. ZAUMEYER 

Office of Experiment Stations 

C. L. LEFEDVRE 

Bureau of Entomology and Plant Quarantine 

WILLIS H. WHEELER 


Office of Information 

ALFRED STEFFERUD, editOT 





foreword 


Ezxa Taft Benson 
Secretary of Agriculture 


To me the most startling aspect of 
plant diseases is that they cost us an 
estimated three billion dollars a year. 

The tragic aspect is that much of the 
loss is a waste that can be prevented. 
Waste is contrary to the laws of Nature 
and the conscience of man. Waste is 
unworthy of a great people. 

To conquer some of the diseases will 
not be easy. New diseases and new 
races of old disease-producing organ* 
isms appear all the time; race 15B of 
wheat stem rust and race i o i of crown 
rust of oats, for instance, appeared just 
when we thought we had rust under 
control. When we extend the produc- 
tion of a crop, the number and preva- 
lence of diseases seem also to grow — as 
has happened to soybeans. 

Some diseases still outside our borders 
remain a threat as communications 
expand. Some (like tristeza disease of 
citrus crops) are new to our country or 
new to a region. Our buying and 
selling of more goods — fruits, vege- 
tables, nursery stock, seeds — enlarges 
the risk of spreading diseases even 
though the facilities for transportation 
and marketing them have been greatly 
improved. 

The cost of materials and equipment 
for fighting the diseases has tiscome 
enormous. And, finally, our efforts 
against plant diseases are made harder 
by the lack of information about them 
among many persons who have to do 
with plants and plant products. 

Nevertheless, I am greatly encour- 
aged by a number ot developments. 
Plant breeders have perfected va- 
rieties of wheat, oats, strawberries, 


and other crops that can withstand, 
for a while at least, the ravages of 
a disease. More effective chemicals 
have been discovered for use against 
the fungi, viruses, bacteria, and nema- 
todes, and others are in process of 
development. Our people give an ever- 
increasing measure of cooperation to 
regulatory procedures designed to halt 
the importation and spread of diseases. 
The handling of perishable foods has 
made great strides in markets, stores, 
and homes in a few years. 

Much remains to be done, however. 

The program I suggest looks toward 
an intensiheation of those efforts and 
greater efficiency in them. 

One requirement is steady, con- 
tinuous research in plant industry, 
geared to immediate problems and to 
the building up of basic knowledge 
that will be of use in solving problems 
of the future. 

We need to give more attention to 
solving permanently the problems of 
plant diseases and not to be satisfied 
with palliatives that at best provide 
only temporary relief. Here, as else- 
where in agriculture, we should be 
mindful of the biological balance — the 
balance of Nature — which our modem 
practices of plow'ing, domestication of 
plants, fast travel, intensive cultiva- 
tion, and clearing of land keep in 
constant jeopardy. Our attempts to 
improve the health of crops have to 
recognize man’s relationship with 
Nature. 

Another requirement is that the 
information be made available to all 
farmers and everyone else to whom 
it will be useful. Historically, the 
function of the Department of Agri- 
culture is to gather information—^o 
research — of value fb fanners and to 
dbseminate that information as widely 
as possible. The lag in time between 
the acquisition of knowledge and the 
time it is made use of often is greater 
than it should be. We need to give 
mbre attention to ways to shorten 
that interval. 

A third facet is the closer integra- 
tion of these new developments into 


vu 



our agriculture and into the segments 
of our national life that are most 
closely tied to agriculture. 

They are part of our goal for Ameri- 
can agriculture: Ample food for all, 
efficient farm production and market- 
higy prosperity for farmers, economy in 


administerii^ sound agricolttiral pro- 
grams, continuing cooperatkm among 
all segments of society. 

This Yearbook, devoted to an im- 
pomnt subject with so many rami- 
hc^ions, will help greatly in the 
achievement of the goal. 



preface 


Alfred Stefferud^ Editor 


This preface may be the proper place 
to answer some questions from readers 
and others about how and why the 
Yearbook of Agriculture is produced. 

The Yearbook of Agriculture is an 
institution older than the Department 
of Agriculture itself. It can be said 
to date from 1849, when the Commis- 
sioner of Patents, the forerunner of 
the Secretary of Agriculture, began 
to devote one part of his annual report 
to agricultural matters. The first 
annual report of the newly named 
Commissioner of Agriculture, issued 
under authority of the act that estab- 
lished the Department of Agriculture, 
was dated 1862. 

The publication of the Yearbook of 
Agriculture is required by the Act of 
January 12, 1895, which provided: 

“The Annual Report of the Secre- 
tary of Agriculture shall hereafter be 
submitted and printed in two parts, 
as follows: Part one, which shall con- 
tain purely business and executive 
matter which it is necessary for the 
Secretary to submit to the President 
and Congress; part two, which shall 
contain such reports from the different 
bureaus and divisions, and such papers 
prepared by their special agents, 
accompanied by suitable illustratioxis, 
as shall, in the opinion of the Secre- 
tary, be specially suited to interest and 
instruct the farmers of the country, 
and to include a ^neral report of the 
operations of the Department for their 
inf6rmation« . . 

The preface to the Yearbook dated 
1894 said: “The present volume repre- 
sents but imperfectly the ideal of what 


such a yearbook should be .... It 
is believed that the character of the 
volume can be improved from year to 
year until it shall become finally a 
standard book of reference for Ameri- 
can farmers. . . .** 

The foundations of the Yearbook of 
Agriculture as it is now were laid by 
Milton S. Eisenhower, its editor from 
1928 to 1935, and Gove Hambidge, 
the editor from 1936 to 1942. Since 
1936 each volume has been devoted to 
discussions bearing on a single field of 
knowledge instead of to miscellaneous 
articles on year-to-year developments 
in a few phases of farming. The sub- 
jects of the new series are: Plant and 
animal genetics, 1936 and 1937; soils, 
193B; human and animal nutrition, 
1939; agricultural economics and his- 
tory, 1940; climate, 1941; diseases of 
animals, 1942; recent developments in 
agricultural science, 1943-^1947; grass- 
land agriculture, 1948; trees, 1949; the 
processing of agricultural prc^ucts, 
*950~*95n insects, 1952. 

For many years the Yearbook of 
Agriculture has been the main (and at 
times the only) means of announcing 
and summarizing comprehensively the 
results of agricultural research — for 
which (as an example) $42,874,000 
was spent in the United States Depart- 
ment of Agriculture in 1952. 

As a book of science, the Yearbook is 
prepared with no thought of influenc- 
ing farm policies inside or outside the 
Department of Agriculture. Its aim 
is to give complete, practical discus- 
sions of one topic in clear (but not 
elementary) language. It is prepared 
primarily for American farmers, but 
changes in the farm population and 
the increasing interest of nonfarm 
citizens in food, clothing, conservation, 
processing, and many other related 
topics mean that they also enter into 
our jconsiderations when we plan and 
prepare a volume. Craps in Peace and 
(1950-1 95 1 ) and Science in Farming 
(1943-1947), for example, dealt with 
many aspects of agriculture of interest 
and value to the whole population. 

Because most of the chapters are 


ix 



later reprinted separately, an attempt 
is made to make each part of the book 
self-contained, even at the cost of some 
duplication. Other than the basic re- 
quirements of accuracy, completeness, 
and propriety, the writers of the 
chapters — who receive no payment for 
their contributions and who include 
employees of the United States Depart- 
ment of Agriculture, research men in 
universities, experiment stations, and 
industry, and other qualified persons — 
are subject to no limitations in the way 
they handle their assignments. 

An editor and an editorial assistant 
comprise the Yearbook staff. 

The Yearbooks of Agriculture, which 
are congressional documents, are dis- 
tributed mainly by Members of the 
Congress. Of the current volume, 
230,850 copies were printed for the 
Congress; 12,000 copies for Depart- 
ment agencies. State and county 
extension agents, and agricultural 
libraries; and about 40,000 copies for 
sale by the Superintendent of Docu- 
ments. Income from the sales by the 
Superintendent of Documents returns 
to the United States Treasury. 

Besides the books themselves, the 
materials in the Yearbooks have even 
wider dissemination — as reprints, in 
magazines, in anthologies, and as 
background for articles in magazines 
and books. Material in the Year- 
books is not copyrighted, and permis- 
sion to reprint it is usually given, with 
certain restrictions, upon request. 

Grass, the 1948 Yearbook of Agri- 
culture, exemplifies the influence the 
Yearbooks have, even though — to 
repeat — our purpose is to report de- 
velopments in research. It was the 
first major publication to recognize 
the importance of grassland farming 
and for many persons was their intro- 
duction to the production and values 
of grass crops. 

The American Institute of Graphic 
Arts chose Insects as one of the ‘‘Fifty 
Books” of 1952 because of its typo- 
graphic design. It was also chosen 
as" an outstanding textbook of 1952. 
No thought, however, is given to 


making 1^; b^ beautiful 

one — it ii |>lai[ined : iu^ 
considerations only . iii. ;inind . The 
cover, for example, is made of the 
cheapest available binding cloth, 
which is impregnated to make it 
resistant to insects and dampness and 
is inexpensively printed over with 
color because otherwise it would be an 
impractical, easily soiled white. 

As to the pre^nt volume: 

Weather, insects, and plant diseases 
often are called the worst natural 
hazards in farming. The 1941 Year- 
book of Agriculture, Climate and Man, 
summarized our knowledge about the 
first. Insects, in 1952, considered the 
second. This book completes the 
triad. 

Some 30,000 different diseases at- 
tack our economic plants — the plants 
grown for sale or use as foods, feeds, 
clothing, and lumber. Others spoil 
or destroy our flowers, shade trees, 
and shrubs. 

In this book we present information 
on the causes and control of many 
diseases of our important crop plants. 
We emphasize practical details, but we 
also discuss fundamental biological 
facts that underlay the comparatively 
new science of phytopathology. 

We use trade names solely to provide 
specific information. Such use does 
not constitute a guarantee or warranty 
of the products named and does not 
signify that the products are approved 
to the exclusion of others of suitable 
composition. 

We hope this volume will enhance 
the prosperity of American agriculture 
and fulfill the expectations of the many 
farmers, Congressmen, students, ex- 
tension workers, and others who have 
written us that a Yearbook on plant 
diseases is needed. 

Many persons have had a part in its 
preparation, and to them ^thanks are 
given. The work of outlining and 
writing the material began in June 
195U typesetting and similar work 
began in August 1 952. The final proofs 
were approved and made up-to-date 
in June of 1953. 



contents 



V 

The Yearbook Committee 

E^a Taft Benson 

vii 

Foreword 

Alfred Steffemd 

ix 

Preface 


COSTS AND CAUSES 


Jessie /. Wood 

1 

Three Billion Dollars a Year 

A. J. Riker 
A. C. Hildehrandt 

10 

Bacteria — Small and Mighty 

C, W. Bennett 

15 

Viruses, a Scourge of Mankind 

L. M, Black 

SS 

How Insects Transmit Viruses 

Russell B. Stevens 

*7 

The Fungi Are Living Organisms 

William W. Diehl 

91 

Identifying a Pathogenic Fungus 

E, C, Stakman 
J. J, Christensen 

35 

Problems of Variability in Fungi 

J. G. Leach 

63 

Bacteria, Fungi, and Insects 

A. J, Riker 
A. C. Hildehrandt 

68 

Crown Gall — a Malignant Growth 

Lake S, Gill 

73 

Broomrapes, Dodders, and Mistletoes 

Albert L. Taylor 

78 

The Tiny but Destructive Nematodes 

Paul R. Miller 

83 

The Effect of Weather on Diseases 

J. E. McMurtrey, Jr, 

94 

Environmental, Nonparasitic Dangers 

George L, McNew 

100 

The Effects of Soil Fertility 

BASES OF CONTROLS 

John C, Dufugan 
S. P. Doolittle 

115 

How Fungicides Have Been Developed 

Jesse R, Christie 

180 

Using Chemicals To Combat Root Diseases 

Walter Carter 

186 

Fumigation of Soil in Hawaii 

Albert L. Taylor 

189 

More About the Control of Nematodes 

R. W, Leukel 

134 

Treating Seeds To Prevent Diseases 

Erwin L. LeClerg 

146 

Making Sure of Healthy Seed 


xi 




L. C, Cochran 
Earle C. Blodgett 
J, Duain Moore 
K. G, Parker 
Donald P, Limber 159 
Paul R. Frink 
Horace S* Dean 


How Nurseries Get Virus-free Fruit Stock 


The Inspection of Imported Plants 
Protection Through Quarantines 


GROWING HEALTHIER PLANTS 


S, A, Wingard 165 
George H, Coons 174 
Frederick J. Stevenson 
Henry A. Jones 


The Nature of Resistance to Disease 
Breeding for Resistance to Disease 

Some Sources of Resistance in Crop Plants 


GRASSES AND LEGUMES 


Earle W, Hanson 
Kermit W. Kreitlow 
Fred R. Jones 
Oliver F. Smith 
Howard W, Johnson 
D, W, Chamberlain 
J, L, Weimer 
J. Lewis Allison 
John R. Hardison 
Howard W, Johnson 
Kermit W, Kreitlow 
Roderick Sprague 
John R, Hardison 
George W, Fischer 
George W, Fischer 
C. L. Lejebvre 
F. L, Howard 
Fred V, Grau 


217 The Many Ailments of Clover 

288 Sources of Healthier Alfalfa 

838 Bacteria, Fungi, and Viruses on Soybeans 

848 Legumes in the South 

853 Leaf Diseases of Range Grasses 
859 Leaf Diseases of Grasses in the South 
868 The Northern Forage Grasses 
867 Root and Crown Rots of the Grasses 
878 Seed Disorders of Forage Plants 
876 Some of the 1 25 Rusts of Grasses 
880 Smuts That Parasitize Grasses 

885 How To Keep Tf’urf Grass Healthy 


COTTON 

Albert L, Smith 898 Fusarium and Nematodes on Cotton 
Lester M, Blank 898 The Rot That Attacks 2,000 Species 

xii 



J. 7*. IhrnsUy 901 Verticillium Wilt of Gotton 
Albert 4 . Smith 903 Anthracnose and Some Blights 
David C. Neal 911 Bacteria and Fungi on Seedlings 
lister M, Blank 919 The Leaf Spots of Gotton Plants 
W. Hardy Tharp 918 Nonparasitic Disorders of Gotton 


FOOD FEED GRAINS 


J. J, Christensen 381 
John H. Martin 

S. C. Salmon ^ 
James G, Dickson 944 
H, H. McKinruy 350 
C. S, Holton 
V. F. Tapke ^ 
/?. W, Leukel 
John H. Martin 
Paul E. Hoppe 377 

Alice L. Robert 380 

Arnold J. Ullstrup 386 

Arnold J, Ullstrup 390 


Root Rots of Wheat, Oats, Rye, Barley 

The Rusts of Wheat, Oats, Barley, Rye 

Leaf and Head Blights of Cereals 
Virus Diseases of Cereal Crops 

The Smuts of Wheat, Oats, Barley 

Four Enemies of Sorghum Crops 

Infections of Corn Seedlings 
Some of the Leaf Blights of Com 
Some Smuts and Rusts of Com 
Several Ear Rots of Com 


VEGETABLE CROPS 


W. J. Z^um^er 
H. Rex Thomas 
W. T. Schroeder 401 
A. G. Newhall 408 
Guy Weston Bohn 417 
J. C. Walker 485 
J. C. Walker 431 
Eugene S. Schultz 435 
Harold T. Cook 444 
Coyt Wilson 448 
S. JP. Doolittle 454 
Huey /. Borders 463 
S, P. Doolittle 466 
Glenn S. Pound 470 
Glenn S, Pound 473 
, Glenn S. Pound 476 
Edmund B. Lambert 
Theodore T. Ayers 


Field Diseases of Beans and Lima Beans 

Root Rots, Wilts, and Blights of Peas 
Blights and Other Ills of Celery 
The Important Diseases of Lettuce 
Cauliflower, Cabbage, and Others 
Hazards to Onions in Many Areas 
Control of Diseases of Potatoes 
The Fungi That Cause Rot in Sweetpotatoes 
Pieventing the Diseases of Peanuts . 

Ways To Combat Disorders of Tomatoes 

Transplants Grown in the South 

Diseases of Peppers 

Diseases of Beets 

Diseases of Carrots 

Diseases of Spinach 

Diseases of the Common Mushroom 
xiii 



John T, Middleton 
Guy Weston Bohn 

m 

Cucumbers, Melons, Squash 

H. Rex Thomas 
W. J, Z^umever 

493 

Developing Healthier Vegetables 

SUGAR CROPS 

George H, Coons 

509 

Some Problems in Growing Sugar Beets 

E, V. Abbott 
P, E, Bouchereau 

5S4 

Rots, Blights, and Leaf Diseases of Sorgo 

E, V, Abbott 

526 

Sugarcane and Its Diseases 

E. V. Abbott 

536 

Red Rot of Sugarcane 

THE TOBACCO PLANT 

E, E. Clayton 

540 

Developments in Growing Tobacco 

E. E, Clayton 

548 

The Genes That Mean Better Tobacco 

J. G. Gaines 
F. A. Todd 

553 

Crop Rotations and Tobacco 

J , G, Gairns 
T. W. Graham 

561 

Soil Fumigation To Control Root Ills 

SOME 

ORNAMENTALS 

W, D, McClellan 

568 

Rust and Other Disorders of Snapdragon 

Kenneth F, Baker 

572 

Fusarium Wilt of China Aster 

D. L, Gill 

578 

Petal Blight of Azalea 

Emil F. Cuba 
Ralph W, Ames 

583 

Infectious Diseases of Carnation 

A, W, Dimock 

592 

Control of Three Ills of Chrysanthemum 

Philip Brierley 

596 

Virus Diseases of the Chrysanthemum 

Robert 0, Magie 

601 

Some Fungi That Attack Gladioli 

Philip Brierley 
Floyd F. Smith 

608 

Virus Enemies of Gladiolus 

Frank P. McWhorter 



C. J. Gould 

611 

Blights of Lilies and Tulips 

W. D. McClellan 

617 

Narcissus Basal Rot 

Wilbur D, Courtney 

621 

Nematodes in Bulbs 

Lu M. Massey 

625 

Four Diseases of Garden Roses 

Philip Brierley 

636 

Viruses on Roses 

L, 0. Kunkel 

642 

Aster Yellows 


xiv 



FRUITS AND NUTS 


G. W. Keitt 

646 

Scab of Apples 

John C. Dunegan 

653 

Blotch of Apples 

Jokn^C, Dunegan 

655 

Bitter Rot of Apples 

D. H. Palmiter 

658 

Rust Diseases of Apple 

A, B, Groves 

663 

Sooty Blotch and Fly Speck 

Roderick Sprague 

667 

Powdery Mildew on Apples 

J. R» Kienholz 

670 

Stony Pit of Pears 

J, R. Kienholz 

674 

Scab on the Pear 

C. Emlen Scoti 

678 

Fire Blight of Pears in California 

A, E, Cavanagh 
C. //. Roihe 

681 

Phony Peach and Peach Mosaic 

John C. Dunegan 

684 

Brown Rot of Peach 

John C. Dunegan 

688 

Scab or Black Spot on Peach 

John C. Dunegan 

690 

Bacterial Spot of Peach 

Donald H, Petersen 

693 

Anthracnose of Peach 

F. H. Lewis 

695 

Cherry Leaf Spot 

H. Earl Thomas 
Stephen Wilhelm 

702 

Two Root Rots of Fruit Trees 

Neil Allan Mac Lean 



E. E. Wilson 

705 

Coryneum Blight of Stone Fruits 

C. N. Clayton 

711 

Root Rots in the East 

L, C. Cochran 
E. L. Reeves 

714 

Virus Diseases of Stone Fruits 

E. E. Wilson 

722 

Bacterial Canker of Stone Fruits 

T. J. Grant 
L. J, Klotz 

730 

The Tristeza Disease of Citrus 

J. M. Wallace 



L, J, Klotz 
J, F. L. Childs 

734 

Foot Rot of Citrus Trees 

J. M. Wallace 
T, J, Grant 

738 

Virus Diseases of Citrus Fruits 

W, B. Hewitt 

744 

Virus Diseases of Grapevines 

Alvin J. Braun 

754 

Ills of the American Bunch Grapes 

W, F. JeJfers 
£). H. Scott 

760 

Red Stele Disease of Strawberry 

Harold E, Thomas 
C. P. Marcus^ Jr, 

765 

Virus Diseases of the Strawberry 

Folke Johnson 

770 

Diseases of Berries in the West 


XV 



IV. F. Jeffers 

775 

Diseases of Berries in the East 

Ausiin C. Goheen 

784 

The Cultivated Highbush Blueberry 

Herbert F. Bergman 

789 

Disorders of Cranberries 

John R. Cole 

796 

Problems in Growing Pecans 

Paul W. Miller 

800 

Filberts and Persian Walnuts 

AFTER 

HARVEST 

G. B, Ramsey 
M. A. Smith 

809 

Market Diseases Caused by Fungi 

Wilson L. Smithy Jr. 
B. A. Friedman 

817 

The Diseases Bacteria Cause 

Lacy P. McCoUoch 

888 

Postharvest Virus Diseases 

Lacy P. McColloch 

886 

Injuries From Chilling and Freezing 

T. R. Wright 

830 

Physiological Disorders 

G. B, Ramsey 

835 

Mechanical and Chemical Injuries 

T. R. Wright 
Edwin Smith 

837 

Cuts, Bruises, and Spoilage 

J. R. Wins/on 
H. B. Johnson 
E. M. Harvey 

848 

Using Chemicals To Stop Spoilage 

John M. Harvey 
W. T. Pentzer 

844 

The Values of Fumigants 

SOME 

OTHERS 

Theodore W. Bretz 

851 

Oak Wilt, a New Threat 

W. A. Campbell 



Otis L. Copeland j Jr. 

855 

Littleleaf in Pines in the Southeast 

George H. Hepting 
Freeman A. Weiss 

858 

Ailments of House Plants 

C. A. Thomas 

863 

Herbs and Other Special Crops 

H. H. Flor 

869 

Wilt, Rust, and Pasmo of Flax 

E. S. Luttrell 

874 

Diseases of Muscadine Grapes 

George A. Zentmyer 

875 

Diseases of the Avocado 

C. E. Williamson 
A. W, Dimock 

881 

Ethylene From Diseased Plants 

E. E. Wilson 

886 

Apricot and Almond Brown Rot 

Frederick L. Wellman 

898 

Some Important Diseases of Cofiee 


897 

Glossary 


909 

Index and color plates 



color 


plates 


FlaU 

Winter spores of wheat stem rust 
Summer spores of wheat stem rust 
Wheat leaf rust 
Wheat stem rust 


Bacterial blight of barley 
Crown rust of oat 
Powdery mildew of barley 
Victoria blight of oat 


Southern leaf blight of corn 
Stunted corn seedling 
Common corn smut 
Corn leaf blight 


Cotton anthracnose 
Bacterial blight of cotton 
Frogeyc of soybean 
Bacterial blight of soybean 


Purple stain on soybean seed 
Purple stain on soybean seedling 
Virus-infected soybean leaves 
Southern blight of soybean 


Virus-infected red clover 
Powdery mildew of red clover 
Leaf spot of Ladino white clover 
Spring black stem of red clover 


Common leaf spot of alfalfa 
Target spot of alfalfa 
Root rot of red clover 
Anthracnose of crimson clover 


Plate 

Leaf spot of smooth brome 
Leaf spot of orchardgraas 
Bacterial blotch of smooth brome 
Leaf scald of smooth brome 


Anthracnose of Johniongrass 
Leaf blight of Sudangrass 
Net blotch of tall Tcscue 
Leaf spot of millet 


Elm phloem necrosis 
Pole blight of white pine 
Oak wilt 
Chestnut blight 


Stem canker on maple 
A fleshy wood-decay fungus 
A toothed wood-decay fungus 
Fungus gall on azalea 


Apple scab on fruit 
Apple bitter rot 
Apple blotch 
Apple black rot 


Apple mosaic 
Apple scab on leaves 
Cedar-apple rust spots 
Cedar-apple rust galls 


Green mold of oranges 
Apple scald 

Stem-end rot of grapefruit 
Phoma spot on tomato 


Black rot of grapes 
Grape downy mildew 
Red stele of strawberry 
Powdery mildew of strawberry 


Brown rot on sour cherry 
Cherry leaf spot 
Peach brown rot 
Pear black spot 


xvu 



Kate 

xvii Peach anthracnoie 

Peach brown rot fungus 
Peach bacterial spot 
Peach leaf curl 


xviii Sour orange scab on lime 
Citrus canker on lime 
Sour orange scab on lemon 
Septoria spot on lemon 


Dollar spot of creeping bent 
Leaf spot of Kentucky bluegrass 
Fairy ring 

Brown patch of Colonial bent 


XX Botrytis blight of tulip 
White stresJc in gladiolus 
Botrytis on tulip bulb 
Bulb and stem nematode on iris 


xxi Virus-infected gladiolus 
Virus-infected tulip 
Virus-infected camellia 
Gladiolus blight 


Golden nematode on potato 
Rose black spot 


Tobacco black shank 
Mosaic and wildfire of tobacco 


Internal cork of sweet potato 
Swcetpotato black rot 
Potato ring rot 
Tuber rot of potato 


Plate 

' Uhriia 
Do^y 

Pea bacterial blight 
Red node of bean 


Bacterial soft rot of lettuce 
Powdery mildew of cantaloup 
Onion pink root 
Altcrnaria leaf spot on tomatoes 


xxvif Pea streak 

Cucumber anthracnose 
Bacterial spot of tomato 
Rib blight of lettuce 


Scab on squash 
Cucumber downy mildew 
Anthracnose of watermelon 
Spinach blight 


Pepper ring spot 
Brown spot of celery 
White rust of spinach 
Peanut leaf spot 


Deficiency diseases of corn 
Magnesium hunger of cotton 


Ergot fungus on orchardgrass 
Fungus fruiting body on ryegrass 
SpK>res and mycelium of a fungus 
Corn smut cultures 


Root knot on tobacco 
A source of resistance to wildfire 


xviii 






Three Billion 
Dollars a 
Year 



costs 


and causes 


Jesm /. Wood 

A farmer in Pennsylvania in 1928 
sprayed his apple orchard with mate- 
rials other than those tested and recom- 
mended by the experiment station. 

The result was an attack of apple 
scab that caused the loss of 80 percent 
of his crop. He had expected 1,500 
bushels, but he got only 300 bushels 
of poor apples. 

Furthermore, the trees were so badly 
weakened that yields in after years 
were low. Because of their poor condi- 
tion, 1 5 percent of the trees died during 
drought in 1930 and 1932. Even 20 
years afterward, the orchard had not 
returned to its original healthy, profit- 
able state. 

But that is not all. The financial set- 
back forced the farmer to borrow heav- 
ily every year in order to produce a 
crop. As a result, the farm became so 
debt-ridden that he would gladly have 
sold it for the amount of indebtedness. 
In order to make a living, he had to 
seek outside employment and finally 
became a full-time worker at another 
occupation. Thus one bad attack of a 
plant disease turned a debt-free enter- 
prise into a liability and wrecked the 
owner’s independence and security. 

Another instance: A farmer had 
owned his farm in Sovith Dakota all his 
life. In 1914-1918, leaf and stem rust 
caused losses to small grains. The yield 
of barley was reduced from 50 bu^els 
an acre to 10; oats dropped from 40 to 
60 to 5 to 1 5 bushels; wheat, from 20 to 
30 bushels to 2 to 5 bushels an acre. 



I 



2 


YEARIOOK OP AOitCULTURI I9S3 


The financial loss amounted to S 1,000 
to $3,000 every year. He borrowed 
money during this time of crop failure. 
A series of years of low prices added to 
his loss. The debt caused foreclosure in 
1934, when the depression was at its 
worst. Here again, a plant disease gave 
the downward push that decided the 
fate of this farm home and business. 

Stem rust caused almost 60 percent 
loss ta the wheat crop in Minnesota 
and some of the neighboring States in 
the epidemic year of 1 935, and the loss 
for the whole country was almost a 
quarter of the crop. Obviously, that 
heavy reduction in over-all yield means 
that some individual fanners sustain 
disastrous losses. Witness the following 
accounts of farm losses due to stem rust 
in Minnesota that year. In one county 
a $4,000 loss was the final factor, al- 
though not the only one, in foreclosure 
on one farm; a $s,ooo loss caused fore- 
closure of a 400-acre farm valued at 
$16,000; a $4,000 loss caused foreclo- 
sure of a 1,100-acre farm worth $25,- 
000. In another county the wheat crop 
on some 700 farms was practically a 
complete failure because of stem rust. 
The average yield expected was ao 
bushels to the acre, but the actual yield 
ranged from nothing at all to about 10 
bushels an acre. The total loss for the 
area amounted to about $400,000. 
Some farmers had to give up farming. 
Others did not recover financial stabil- 
ity for 5 years or longer. 

About 50 years ago Granville wilt, or 
bacterial wilt, of tobacco appeared in 
the southern part of Granville County, 
N. C., where about 7,000 acres of to- 
bacco were grown on 1,500 farms, 
some of which had been owned and 
operated by the same families for gen- 
erations. The disease increased until it 
was destroying 20 to 50 percent of the 
crop each year; the loss amounted to 1 
to 2 million dollars annually. All at- 
tempts at control failed. Many farmers 
lost their farms, farmsteads deterio- 
rated, the morale of the farm population 
dropped, education was neglected, and 
living and health standards went down. 
Between 1920 and 1940, when the dis- 


ease was at its peak, total loss firom 
forced sale of farms at sacrifice and 
from reduced tobacco production 
amounted to 30 to 40 million dollars. 
All this because of one plant disease in 
one conununity! 

Those are some of die cases reported 
by county agents. By no means were all 
of the losses involved so severe or lasting 
as the ones I have mentioned, but in 
many other instances a plant disease 
was partly or wholly the cause of seri- 
ous difficulty. 

Possibly you think that those fanners 
must not have been very skillful, that 
good farmers would not have such 
trouble, or perhaps that they did not 
arrange their financial affairs very 
well. Undoubtedly poor judgment 
was a factor sometimes. 

But how about the tobacco farms 
that had been owned by the same 
families for generations, only to be 
lost when a disease-producing organ- 
ism invaded the soil and could not be 
checked? The obvious remedy would 
have been to change to some other 
main crop or to vary the crops, but a 
region that is suited to and derives 
most of its income from one major 
profitable cash crop does not easily 
make such a change, for the change 
involves much more than just growing 
a different plant. Moreover, the bac- 
terium that causes the disease has so 
many other hosts besides tobacco (such 
as peanuts and potatoes) that a change 
might merely from the frying pan 
into the fire. 

With farming, as with any occupa- 
tion, many factors make success or 
failure. The farmer, no less than every- 
one else, is affected by the ups and 
downs of the general economic situ- 
ation. In good times he can absorb 
or recover quickly from large losses 
due to any cause. In hard times small 
losses may be enough to shove him 
under. As for the individual economic 
circumstances, good financial manage- 
ment is as essential as fertile soil and 
skillful farming to be sure of leeway 
enough to meet all chances of loss and 
take advantage of all opportunities for 



rHtil •ILitON DOilAIS A YfAl 3 


profit. Lacking any of thow qualiti^ 
a former may bardy make a living in 
the best of times and co'tainly will 
possess no margin for emergencies. 

Granted all that: The fact remains 
that agriculture is subject to numerous 
risks^ which at times endanger the 
prosperity of the best of farms or of 
whole areas. Economic fluctuation is 
just one of the risks; it is shared with 
the rest of the population. Others are 
peculiarly agricultural. Among them 
are plant diseases. 

Weather, insects, and plant diseases 
are the three great natural hazards of 
crop production. Weather is perhaps 
the chief one, but the order of impor- 
tance is not certain. The interrelation 
between weather and disease and be- 
tween weather and insects and among 
all three, if the disease is caused by a 
pathogen carried by insects, is so close 
and so complicated that sometimes it 
is hard to determine the actual origin 
of trouble. Moreover, the causes of 
plant diseases are mostly not obvious 
to the unaided eye, and loss due to 
them may be ascribed to the more 
conspicuous insects or to weather con- 
ditions. In any event, the relative 
importance of the three factors is not 
fixed but varies according to season 
and location, and the total loss due to 
each one by itself is quite enough to 
make comparisons superfluous. 

In the United States the average 
annual loss from plant diseases is esti- 
mated to be about 3 billion dollars. 

We have no way of establishing a 
precise figure, and this one is based on 
many assumptions but it could well be 
under rather than over the real 
amount. Without the control measures 
we do have, the loss would be much 
greater. 

The primary reason for the impor- 
tance of plant diseases or of anything 
else that affects plant growth is not 
economic. We arc apt to take green 
plants for granted, so familiar are they. 
We use them for i6od and clothing and 
many other things, but we seldom 
consider their true significance. Green 
plants alone, save for some negligible 


exceptions, manufacture the basic ma- 
terials of life; the existence of animab 
and human beings depends on the 
products of these living green factories. 
Any serious breakdown in this process 
threatens life itself. The famines that 
periodically cause so much suffering 
in many regions of the earth prove 
that. 

Disease is a cause of lowered efficien- 
cy or final bre^down in the plant’s 
functions. Disease, literally “dis-ease,” 
is one of those terms that are very hard 
to define although everybody knows 
what they mean. Broadly speaking, it 
means disturbance in functioning and ^ 
growth. Whethei the result of the * 
disturbance is important to us depends 
on how seriously it affects the yield 
and quality of the product for which 
we grow the plant. 

The reaction of the affected plant to 
the cause of the disturbance produces 
the various symptoms and conse- 
quences that we recognize as the effects 
of disease, such as wilting, dicback, 
root and stem rots, damping-off, can- 
kers, witches’-brooms, stunting, un- 
thriftiness, poor yields, shriveled ker- 
nels, blighted, spotted, discolored, or 
deformed foliage, fruit decay, and the 
numerous other rnanifestatiom of ab- 
normal condition. 

Diseases do not just happen; they arc 
always the result of a cause. The causes 
are divided into two broad groups, 
namely, parasitic and nonparasitic. 

Man and animals are not the only 
kinds of life that depend on green 
plants for their existence. Minute 
living organisms may invade and grow 
within the plant tissue and obtain their 
nourishment from it. Sometimes the 
involuntary plant “host” tolerates the 
existence of the invader within its body 
without apparent hami; in fact, there 
may be a reciprocal benefit from the 
association. More often the parasitic 
organism interferes with the function- 
ing of the affected plant, and disease is 
the result. From our point of view 
that is bad, although the parasites are 
doing the same thing we do — living 
on the products of the green plant. 



YiAitOOK OP AOilCUtTUti If 93 


But they both lodge and board with 
their host, as it were, and they cat at 
the first table. We get what is left. 
Sometimes that is a very small portion. 

Parasites that cause diseases through 
their growth in their hosts are called 
“pathogens,” and the diseases that 
they cause arc “pathogenic” or “para- 
sitic” diseases. Because in this instance 
the diseases are the result of infection 
that may spread from plant to plant, 
they arc also called the “infectious” 
diseases. 

The nonparasitic diseases arc due to 
environmental or nutritional factors 
^unfavorable to the plant, or to some 
abnormality in the constitution of the 
plant itself. Nutritional disturbance 
arising from either actual deficiency or 
unbalanced supply of essential nutrient 
materials is one of the most important 
causes of plant disease. Deficiency 
diseases are some of the most common 
manifestations. In some places they 
are recognized as the most troublesome 
plant disease problems, and it has been 
suggested that they arc probably much 
more widespread and significant gen- 
erally than has been realized. 

Our cultivated plants have inherited 
disorders from their wild ancestors, 
which are just as susceptible as any 
garden, field, or orchard crop to the 
attack of parasites or to functional 
disturbance from other causes. Culti- 
vation, however, greatly increases 
liability to the development of disease, 
for various reasons. 

Although many parasitic organisms 
can attack a great number of different 
kinds of plants, most of them are 
limited to a narrow range — sometimes 
to only one species. On anything else 
they cannot grow. 

In the wild, mixed growth of differ- 
ent kinds of plants as well as natural 
genetic variation in the susceptibility 
among individuals of the same species 
interpose many barriers to the spread 
of pathogens that attack any one host. 
But in cultivation numerous individ- 
uals of one kind of plant grow near 
cjach other, ’sometimes almost to the 
exclusion of any other plant over a 


wide area. Moreover, cofUKt^y w 
unconsciously, from the very bepnnlng 
crop plants have been selected for 
propagation on the basis of c>ne or 
another desirable characteristic, so 
that much of the original divenity is 
lost, including variation in reaction to 
pathogens. Both the chances for infec- 
tion and opportunities for spread by 
a pathogen to which a crop is vulner- 
able are therefore multiplied enor- 
mously. 

Not only does cultivation enhance 
the local spread of disease. Crops 
accompany agriculture to new loca* 
tions far distant from their original 
homes. If a pathogen is equipped to 
survive the journey and if it meets 
with favorable conditions in the new 
location, it, too, will become estab- 
lished in the new home. 

But that is not all. Wild plants in the 
new place may harbor a pathogen 
that finds the introduced crop much 
more vulnerable to its attack than its 
original hosts. For example, the fire 
blight bacterium is indigenous to this 
country, where it infects various 
native plants of the rose family with- 
out damaging them too much. Pear 
trees and apple trees brought by 
settlers from abroad proved to be 
very susceptible; pear trees arc so 
susceptible that it is hard to grow 
them in many parts of the country. 
Many “new” diseases appear in this 
manner; they arc not really new but 
are transferred from wild plants. 

Many a plant pathogen is inconspic- 
uous in its effects until it meets a 
susceptible new host or perhaps until 
it acquires an efficient vector. 

The susceptible new host may be a 
native plant and the pathogen an 
introduced organism. Plant disease 
problems are by no means limited to 
cultivated crops. When a wild species 
has rather uniform susceptibility to 
one kind of pathogenic organism, disease 
caused by the organism can become 
as destructive in natural stands as, for 
instance, rust is in the occasional wide- 
spread outbreaks in wheatfields of the 
Great Plains. Wild stands may suffer 



THiti •UlfON DdiiAiS A YiAl 5 

eveii grea^ destruction, in fact, be- of disease must often have caused 
cause ordinarily there is not much famine and suffering long before the 
that can be done to stop the spread of beginning of recorded history, 
the pathogen. Ancient literature contains many 

The chestnut blight is an example, accounts of maladies in crops. Among 
In the 40 years or so since it was first them we can most surely identify the 
discovered in this country, the disease rust (probably both leaf and stem 
has practically killed the American rust) of grains. From our own experi- 
chestnut throughout its native range, cnce with its destructiveness we can 
If it were not for cultivated plantings understand the ancient fear of the 
and artificial hybridization with other disease. It was all the more frighten- 
species of chestnut, none of the good ing lx;caiisc its origin was a mystery, 
qualities of this useful, handsome tree Its appearance in the fields was 
could be preserved for future genera- believed to evidence of the dis- 
tions. Who knows how often similar pleasure of the gods. The early Ro- 
events in ages past may have caused mans called the disease “robigo” be- 
other potentially valuable plants to cause of the redness. Their corn god, 
become extinct? Robigus, was named for the rust. 

Good nutrition is as necessary to This god, in their belief, possessed 
the health of plants as it is to ours. In the power of inflicting or withholding 
the natural environment, materials the scourge of the disease. He was 
used by the plant in its growth are so important to them that every 
constantly being returned to the soil year they staged a festival, the Rubi- 
in decaying plant debris, but usually galia, in his honor with ceremonial 
in cultivation they arc removed from offerings and sacrifices to ward off 
the soil, and little sucli natural replace- his displeasure and seek his favor for 
ment takes place. Moreover, even in the crops. 

the natural state some soils lack one We know now that there is nothing 
or another of the essential elements, mysterious about the causes of plant 
and cultivation may increase the dc- diseases. Even if we have not yet 
ficicncy to the danger point. The been able to discover with modern 
amount and particular combination techniques the origin of some one 
of nutrients required for proper growth disease, we know that it has a natural 
vary according to the kind of plant, explanation. Modern knowledge has 
and crops will not always 1^ planted removed the fear of the supernatural 
in soils suited to their special needs, and it has given us the means by which 
Maintaining and improving the sup- to reduce the damage due to plant 
ply of plant foods in the soil to prevent di.sea$cs, but it h£is not made them 
the development of nutritional disca.se less important. 

is a major agricultural problem. One of the most tragic events in 

Thus, when men first thought to history led to the beginning of real 
insure themselves of a food supply by knowledge about plant diseases and 
growing instead of gathering it, they to the development of the science of 
encountered difficulties. They wore plant pathology. That was the Irish 
out their soil — and then more soil, as famine in the middle of the nine- 
they abandoned their clearings and teenth century. Two^circumstances 
moved on to fresh land. Along with were responsible. First, the impover- 
the plants they found most u.seful, they ished population had become almost 
unwittingly started cultivating the wholly dependent on their potato 
parasites that affected their chosen gardens for food. Second, the potato 
crops. It was ages before they rccog- crop for 2 years, 1845 and 1846, was 
nized the cause or could combat the almost wholly destroyed by late blight, 
trouble from the parasites. The failure Accounts .of physical misery and 
of staple crops due to severe outbreaks spiritual anguish suffered because of 



YEAtBOOK OP AOtlCillTVtl 19Sa 


6 

the devastation caused by this one 
disease go far beyond anything that 
ordinary experience equips one to 
understand. Ireland lost almost a 
third of its population between 1845 
and i860 as a direct result of the 
outbreak of late blight. A million 
people died from starvation or from 
disease following malnutrition. A 
million and a half more emigrated. 

The outbreak in Ireland was part 
of a pandemic — that is, the disease 
suddenly became widespread and 
destructive almost simultaneously in 
several European countries and in 
the United States as well. As far 
as can be determined, the disease 
had appeared in these regions not 
more than 2 or 3 years previously. In 
the meantime, the pathogen evidently 
increased and became widely dis- 
tributed, so that when the weather 
became generally and extremely favor- 
able, as happened during the years 
of the pandemic, it could attack 
rapidly and in force over a wide 
area at once. 

Why did this outbreak overwhelm 
the Irish and affect other peoples 
much less? The answer is not simple. 
It lies partly in agricultural and 
partly in political history. But es- 
sentially it is that miserable economic 
conditions led to the almost sole 
reliance of the Irish peasantry on the 
easily grown, productive, and filling 
potato for their main food. 

In other places food resources were 
more varied so that destruction of 
the potato crop did not have any- 
where near the same importance. 

Even after this grimmest of epidemics 
abated, its consequences remained. 
The disease had become a fixture in 
potato culture. It was more or less evi- 
dent almost every year, and serious 
outbreaks, although none again so dis- 
astrous as the great pandemic, oc- 
curred from time to time whenever the 
weather was favorable. The tragic 
drama of the famine was one of the 
decisive factors in subsequent social 
and economic policy. Its influence on 
3ntish-Irish relations is still felt. 


Of course, circumstances must be 
unusual indeed for such extreme dis- 
aster to be caused by the attack of a 
plant disease, or, for that matter, by 
anything else. There have been other 
records of famine due to the severe 
occurrence of a plant disease. In 1 733, 
more than a century before the Irish 
famine, 12,000 persons on one Japa- 
nese island died because of failure of 
the rice crop, caused perha}>s by stunt, 
a virus disease. Early settlers in Aus- 
tralia are said to have suffered more 
than once from lack of food because 
their grain crops were destroyed by 
leaf rust. Actually, however, except for 
the toll of human lives that make it so 
terrifying and so impressive, famine is 
comparatively minor as an expression 
of the importance of plant diseases. 

Nowadays help can reach victims 
quickly almost anywhere in the world, 
and there is less and less likelihood of 
famine or excuse for it. 

That last statement is true except 
in times of stress and emergency, of dis- 
rupted transportation and world up- 
heaval. Late blight is said to have had 
a place in the defeat of Germany in the 
First World War. In 1917 it destroyed 
about a third of the potato crop, which 
made up a large part of the wartime diet 
of the Germans. Reduction in the al- 
ready scanty food supply contributed 
to the breakdown in morale and physi- 
cal endurance that led to the end of the 
war. Here again, this required a favor- 
able combination of circumstances; 
seldom docs a single plant disease 
influence military affairs to that extent. 
Plant diseases can cause or aggravate 
serious shortages in wartime, however, 
all the more so because then fewer 
varieties of crops are apt to be grown; 
their products, whether for food or 
other consumption, arc urgently needed 
in greater quantity than usual; replace- 
ments or substitutes arc hard to get or 
arc unsatisfactory; diversion of chemi- 
cals necessary in the manufacture of 
fertilizers and fungicides to other use 
hampers control of parasitic and non- 
parasitic diseases; and the overloaded 



THREE EliLION DOUAR5 A YIAR ' 


tramportatlon facilities multiply the 
effects of all the other factors. 

Diseases that bring illness and death 
to persons and animals that eat af- 
fected products make up a special cate- 
gory, of which ergot is a good example. 
Ergot is caused by a fungus that infects 
the flowers of many grasses, including 
the cereal grains, and replaces the seed 
kernels with its sclerotia. The sclerotia 
contain alkaloids, which have a power- 
ful action on the nervous system and 
can produce gangrene or convulsions 
and, in severe cases, death. 

Rye is especially susceptible to ergot. 
Ergotism in man is associated mostly 
with its use. In most of Europe through- 
out the Middle Ages rye was the main 
food cereal for many people. Many 
severe epidemics of ergotism cost thou- 
sands of lives. Later, after the cause 
became known, standards for permis- 
sible ergot content of grain made ergot- 
ism rare in human beings. In years 
favorable to ergot attack, infected 
grass in pastures frequently causes con- 
siderable loss of livestock, however, 
especially from abortion. 

There is another side to the ergot 
story. The specific action of its alka- 
loid content makes it valuable in 
medicine, particularly in childbirth. 
Ergot is supplied commercially from 
several regions where the climate reg- 
ularly favors it. Elsewhere in epidemic 
years a rye crop may be more valua- 
ble for its ergot content than for the 
grain. In regions where tlic climate 
does not favor ergot, or in times of 
scarce production, or when the regu- 
lar source is shut off for some reason, 
artificial infection of rye fields is re- 
sorted to in order to obtain a supply 
of the drug. The only ergot used in 
medicine is rye ergot. 

'Tntoxicating bread,** which pro- 
duces symptoms of weakness, vertigo, 
headache, and nausea, results when 
bread is made from rye grain heavily 
infected with one or more species of 
Fusarium. Cases have occurred in 
Europe from time to time. “Scabby** 
grain, due to Gibberella zeae^ so preva- 
lent in warmer parts of our humid 


eastern and central region, is toxic 
to hogs; in epidemic years consider- 
able loss of hogs follows feeding of 
infected grain, especially barley. 

It should be noted that among all 
the thousands of plant diseases only 
an infinitesimal number cause harm to 
animals or man in any way other than 
through their effect on abundance of 
wanted products. 

Famine and war shortages, tragic or 
dangerous though they may be, are 
really special instances of the import- 
ance of plant diseases. So also is 
toxicity to human beings or animals. 
Usually, the consequences are not so 
impressive, although in the long run 
they are as significant. 

Many times in history a thriving 
agricultural industry has been threat- 
ened or destroyed because of severe 
losses caused by plant diseases. Recov- 
ery of a region so affected may be diffi- 
cult. It involves, among other things, 
the search for an acceptable substi- 
tute crop equally adaptable to the 
region and equally profitable, the 
learning of new techniques of cultiva- 
tion and handling, and the develop- 
ments of markets for disposal of the 
new product. The change may never 
be completely successful, and as a 
consequence agriculture deteriorates 
or is abandoned. Perhaps soil or cli- 
mate is particularly suited to the origi- 
nal crop and no other proves econom- 
ically successful; or markets for an 
otherv\’isc suitabk crop may already 
be preempted; or the new crop itself 
be attacked by devastating disease. As 
a rule, a readjustment means much 
hardship for the affected populations, 
either the farmers themselves or per- 
sons otherwise employed in the crop 
industry. 

Before 1870 Ceyloif was preeminent 
in coffee production. The coffee rust 
fungus, a native parasite on wild 
coffee trees, about that time invaded 
the plantations and could not be con- 
trolled. The disease spread throughout 
the East, and yields dropped so low 
that the industry could not maintain 
itself. South America, particularly 



8 


YEARIOOK OF AGRICULTURE 1953 


Brazil, thereupon became the coffee 
empire of the world. Ceylon planters 
started growing the tea bush. 

In our own country the virus disease 
peach yellows was a main factor in the 
great reduction in peach culture in 
Michigan, Maryland, and Delaware 
in the i8oo’s. Commercial production 
of peaches has never again attained the 
same importance there. In Berrien 
County^ Mich., between 1874 and 
1884, the acreage in peaches dropped 
from 6,000 to 500 acres, and the num- 
ber of peach trees from 654,000 to 
fewer than 55,000. In 1920 there were 
only one-tenth as many peach trees in 
Delaware as there had been in 1890. 
In Maryland, Kent and Queen Annes 
Counties grew nearly one-half of the 
peaches in 1890, but only 5 percent in 
1920. Not only did the disease ruin 
the peach industry; it also lowered the 
value of the farms. Before the onset of 
the disease, peach farms in northern 
Delaware sold for high prices, but 
usually paid for themselves in peaches 
wHhin a few years. Fifteen or twenty 
years later they dropped to 50 to 80 
dollars an acre. In some regions where 
the disease was most destructive, it was 
hard to sell a peach farm at any price. 

The important sugar beet industry 
in the Western States was almost de- 
stroyed by another virus disease, curly 
top, until, after long research, resistant 
varieties were developed. An additional 
protection to both sugar beets and the 
many other plants affected by the 
curly top virus is the spraying of 
Russian-thisde, the most abundant 
overwintering host of the insect vector, 
with some of the new weed killers. 
Without resistant varieties and reduc- 
tion in vector movement, curly top 
would make it impossible to produce 
profitably many of the important vege- 
tables, including beans, tomatoes, and 
muskmelons, which harbor it. 

One could go on and on, multiplying 
illustrations, big and little, temporary 
or permanent, past or present, mere 
threat or realized actuality. For in- 
stwee; Pierce s disease of grapes in 
California; the deficiency diseases, 


which are particularly important in 
Western States; cranberry false blos- 
som in New Jersey; fire blight of pear, 
because of which “one of the greatest 
industries of the San Joaquin valley 
vanished like a dream”; verticillium 
wilt of cotton in the £1 Paso region of 
Texas; melon mosaic in the Imperial 
Valley of California; and many, many 
others throughout the world. 

The essential feature of plant diseases 
however, is that they deprive every- 
body, not just farmers, ^ the plant 
products they destroy. The loss from 
all diseases of all crops is estimated to 
be about 10 percent. That is an aver- 
age. Some crops suffer more loss. 

Others are negligibly affected. The 
importance of any one disease depends 
on the value of the crop it attaclu, the 
severity of the disease, and the ease 
with which it can be controlled. 

Diseases that attack the basic food 
crops, such as the cereals, esp>ecially 
wheat and rice, potatoes, or others 
according to regional use, arc natur- 
ally of greatest concern. Epidemics in 
the chief centers of production can 
cause scarcity, with serious national 
and (especiily with wheat) inter- 
national consequences. Prices of the 
affected crop and of substitutes for it 
go up.' The amount of money available 
for buying other commodities, whether 
food or any other, consequently is 
reduced. It is a chain; in our present 
national and world economy, no part 
is separate. 

For the farmer, the labor employed 
in planting and caring for an affected 
crop is wasted; income is reduced and 
uncertain. With epidemic diseases he 
may suffer disastrous losses in one 
season. The less variable diseases take 
a constant toll that in total amount 
could make the difference between 
success or failure in the long run. The 
lesser continual losses may be sufficient 
to prevent the building-up of reserve 
for emergencies, and thus the effects of 
the occasional severe losses are en- 
hanced. The instability of farm income 
is reflected in the national condition. 

Losses from plant diseases do not 



THtll ilillON 

Stop when a product is harvested. Fruits 
and vegetables spoil in transportation, 
maHcet, and storage. Infection may 
have started in the field before harvest 
cm: it may be acquired later. Whatever 
the reason, the food supply is further 
depleted and food bills increased. 

If all the waste that is due to plant 
disease could l:>e prevented, it would 
mean an increase of lo percent over 
our present crop production, or, alter- 
natively, io j>ercent of the land could 
be used for other purposes or lo per- 
cent of the farm population could 
engage in other occupations and we 
would still have as much of every- 
thing as we do now. 

It should be emphasized that the 
plant disease situation is always chang- 
ing. No matter how up-to-date our 
information, there is always some new 
problem to meet. Pathogens are as 
variable in genetic make-up as any 
other organism, and new races al>lc 
to attack hitherto resistapt varieties 
of crop plants appear frequently. 
Wheat stem rust is a familiar e.xample. 
A crop variety developed for resist- 
ance to one pathogen may be ex- 
tremely susceptible to a previously en- 
tirely negligible organism, as hap- 
pened with the Victoria blight of oats 
that flared up so suddenly and de- 
structively a few years ago. Pathogens 
arc carried from country to country 
in many ways and in spite of careful 
precautions. Examples of entry into 
this country arc numerous. White pine 
blister rust is one; potato bacterial 
ring rot is another. 

A grower, noting .iccounts of disease 
causing devastation in some other 
place, may consider that it has noth- 
ing to do with him, since crops in his 
region or his own fields arc not 
affected; perhaps, even, he may re- 
joice becau.se scarcity has led to higher 
prices for his own crop. Temporarily 
he may be justified, but it is never 
safe for growers or regions to be com- 
placent a)x)ut freedom of their crops 
from plant diseases. To maintain this 
position takes constant watchfulness4 

If nothing could be done about 


DOLLAIS A YIAt 9 

plant diseases, there would be little 
point in discussing their importance. 
We would just have to take the loss 
and get along with it as best we could. 
We have seen how that works in some 
instances. We do know how to re- 
duce the effects of most diseases, how- 
ever. I ncreasing ottr knowledge about 
diseases will allow us to gain more 
advantage over „them, provided we 
make use of it. The available control 
measures are not nearly so often or so 
efiiciently used as they should be. 

Diseases are too often taken for 
granted until irreparable harm has 
been done. Recognition of their im- 
portance is the first step in doing 
something about them. 

Jessie I. Wood is an associate pathol* 
ogist in the Plant Disease Survey^ Bureau 
of Plant Industry^ Soils, and Agricultural 
Engineering. She obtained her masters 
degree from Stanford University in rgrS 
and has been with the Department of Agri* 
culture since igtg. 



Ergot sclerotia and barley kernels. 



lO 


YEAHIOOK OE AOtlCUlTUM 1959 


Bacteria — 

Small and 
Mighty 

A. J. Rikeiy A. C. Hildebrandt 

Bacteria are an important part of the 
world we live in. They cause diseases 
in man such as tuberculosis, typhoid 
fever, and diphtheria, and in animals 
such as anthrax, brucellosis, and swine 
erysipelas. 

Many bacteria are helpful to man. 
They produce useful food and chem- 
icals, assist in the decomposition of 
wastes, and increase the fertility of soil. 
Most of us know al)Out the nitrogen- 
accumulating root nodules of clover, 
alfalfa, and other legumes that arc pro- 
duced by bacteria growing in their 
roots. 

Bacteria also cause disease in plants. 
The discovery of this role was made 
only 75 years ago. After Louis Pcistcur 
proved that bacteria could produce 
animal disease, Professor T. J. Burrill. 
of Illinois, who studied in Europe, was 
filled with enthusiasm. He began work- 
ing with a devastating disease of 
unknown cause that was sweeping 
through pear and apple orchards of 
the Midwest. Burrill soon proved that 
the disease, which we now call fire 
blight, was caused by bacteria. Bur- 
rill’s brilliant pioneer discovery swept 
aside earlier speculation and igno- 
rance. It pointed the way. 

But BurriU’s work did not go unchal- 
lenged. Many doubters spoke up. Dr. 
Erwin F. Smith and his associates of 
the United States Department of Agri- 
culture carried the work forward 
meanwhile. They overcame the oppo- 
sition, scorn, and derision of some 


distinguished scientists. They proved 
that bacteria caused many plant dis- 
eases. Such researches provided a solid 
background for later work on plant 
bacteria. In his book, Buclerial Diseases 
of Plants^ in 1920 Dr. Smith summa- 
rized much of this work. Now we have 
knowledge of more than 1 70 different 
kinds of bacteria, which cause diseases 
in flowering plants belonging to 150 
genera of 50 families. 

Among living agents that cause plant 
disease, bacteria are perhaps the small- 
est (if we do not consider viruses as 
living). Such bacteria are so minute 
that about 25,000 laid side by side and 
8,000 to 12,000 laid end to end would 
not measure more than an inch. 

Each bacterial cell is an independent 
plant. If many cells adhere to one 
another, a mass of cells may be formed. 
But each cell acts independently; the 
numbers of bacterial cells increase 
when the cells cut themselves into two. 
They multiply by dividing, so to speak. 
Under favorable circumstances repro- 
duction by cell division may happen as 
often as three times in an hour, and 
enormous numbers of bacteria may be 
produced within 2 or 3 days. 

The large surface area of each bac- 
terial cell and the myriad bacteria 
usually-present in diseaiscd plants give 
the invading bacteria a great advan- 
tage as they attack the cells of the 
affected plant. That helps to explain 
the rapid progress a bacterial disease 
makes under favorable conditions. 

Their survival depends upon their 
ability to utilize the living or dead 
organic compounds they find in their 
host plants. 

The bacteria that live only on dead 
animal or plant remains are termed 
saprophytes. Those that produce dis- 
ease are parasites or pathogens. Many 
kinds of bacteria, especially those that 
cause plant disease, can live cither as 
parasites or as saprophytes. Many dis- 
ease-inciting bacteria are able to over- 
winter or maintain themselves between 
successive susceptible crops by living 
saprophytically on plant refuse. 

Bacteria that induce plant disease 



• ACTIIIA — SMALl AND MfOHTY 


M 


are all mare or less short and cylin- 
drical. They are dei»:ribed as rodlike. 
None is spherical like the coccus forms, 
which cause some animal or human 
diseases. Some species of plant para- 
sitic bacteria have one to several fila- 
mentous, or hairlike, motile append- 
ages called flagella, which they can 
wave or vibrate and by them move 
for short distances in water or in plant 
juices. Some kinds have flagella at one 
end or both ends (polar). Still others 
develop flagella at many places on the 
surface of the cell (pcritrichous). Even 
those that have no flagella, however, 
can be carried rapidly from one place 
to another by flowing or splashing 
water, or by wind-blown droplets of 
moisture, by insects, and by various 
agricultural operations. Any agricul- 
tural practice that involves the trans- 
port of soil may serve to carry path- 
ogenic bacteria from one place to 
another. 

Bacteria gain entrance to plants 
through uninjured tissue, natural open- 
ings, and wounds. The potato scab 
organism usually enters through len- 
ticcls but can penetrate thin-skinned 
tubers directly. Root-nodule bacteria 
of legumes enter through the fragile 
root hairs. The small natural openings 
to the atmosphere — the stomata — in 
the leaves are the ports of entrance for 
the bacteria that cause angular leaf 
spot of cotton. The bean blight organ- 
ism may also enter through stomata. 
I'he bacteria that cause blackleg of 
cabbage enter through hydatlu^es. 
Hydathodes, if present, usually are 
found at the edge or tip of the leaf. 
They are specialized gland ceUs that 
excrete fluids. Often under highly 
humid conditions small droplets of 
water may be seen adhering to the 
margins of such leaves as cabbage or 
wheat. As these droplets later may be 
resorbed by the leaf, any bacteria 
present may also gain entrance and 
multiply. The bacteria that cause Are 
blight of pear, apple, and quince trees 
enter at blossomtime through the 
specialized cells of the flower that 
produce nectar. This type of environ- 


ment is especially suited to the 
and multiplication of the parsuates. 

Many kinds of bacteria enter their 
host plants through wounds. Those 
producing sott rots of fruits and vege- 
tables and the species causing crown 
gaU of several kii^s of plants are good 
examples. 

Once bacteria get into a plant, some 
may be carried along with or hiove in 
the sap stream. Others may move short 
distances in plant juices by swimming 
or be pulled about by the movement 
of fluids between or in the cells. 
Capillary attraction and changes in 
the pressures and tensions on fluids 
that sometimes flood the spaces be- 
tween the cells move the bacteria from 
place to place inside the plant. Flood- 
ing of plant tissues often increases as 
certain bacteria withdraw liquid from 
plant cells bordering the invaded 
tissue. In early stages of disease, bac- 
teria commonly develop in the spaces 
between the cells. But as cell walls are 
injured and the cells of the plant are 
killed, they become perforated by 
bacterial action. Then the bacteria 
may penetrate inside the cells and 
continue the disintegration. 

Plants respond in many ways to 
invasion by bacteria. Their response 
often is so specific that the disease they 
cause can be identified by the symp- 
toms. Among the symptoms of bacte- 
rial infections are galls, wilts, slow 
growth, dwarfing, imperfect fruits or 
ears, rots, color changes of various 
plant parts, retarded ripening, dis- 
tortion of leaves, cankers, brooming, 
fasciation, and leaf spots. Rots may 
be either localized at one place or in 
one tissue or may involve the whole 
plant. Wilts generally affect the whole 
plant. Galls often affect only a part of 
the plant. ^ 

Among the many well-known bac- 
terial galls are olive knot, cane gall, 
beet pocket rot, sweet pea fasciation, 
hairy root, and crown gall. All contain 
large swollen cells and small, rapidly 
dividing cells along with vascular 
cells in a relatively disorganized ar- 



12 


YEAISOOIC OF AOtlCULTUti 1FS3 


rangexnent. Eventually these gall struc- 
tures may interfere with the normal 
transmission of water and food sup- 
plies, and the plants may wilt and die. 
Of these, crown gall, which has a 
very wide host range, has been studied 
extensively because of the opportunity 
it provides for clarifying basic prin- 
ciples of diseased growth as a biolog- 
ical phenomenon. The legume root 
modules are usually beneficial through 
their ability to fix nitrogen. They have 
great economic importance. 

Bacterial wilts may be quite destruc- 
tive — for example, in sweet corn, 
cucumber, tobacco, and related plants. 
Such bacteria may produce a slime, 
which helps to plug the water-con- 
ducting tissue of the invaded plant. 
Closely related are such diseases as 
black rot of cabbage, ring rot of 
potatoes, and tomato canker, which 
may start in the water-conducting 
tissue but subsequently result in 
disintegration of surrounding tissue. 

Cankers develop from the extensive 
tissue destruction, for example, by the 
fire blight bacteria, or from the lesions 
of the tomato canker organism. This 
latter bacterium may produce only 
local spots on the tomato fruits. Thus 
the symptoms induced by one organ- 
ism may be quite different, depending 
on the plant tissue infected and other 
variables. 

Local spots occur most commonly on 
the leaves, but sometimes appear else- 
where, as, for example, on many fruits. 
Symptoms of black arm of cotton show 
when the angular leaf spot bacteria 
enter the stem and girdle it. Bacterial 
blight of beans, halo blight of oats, 
potato scab, and many o&ers appear 
prinwrily as local spots. The bacteria 
causing halo blight of oats and wildfire 
of tobacco produce toxic substances 
that are responsible for the yellowish 
areas immediately around ^e dead 
spots where the bacteria have invaded 
the tissue. 

^ Soft rots develop in relatively fleshy 
tissues when certain bacteria invade 
them extensively. Such bacteria pro- 
duce an enzyme that dissolves ^e 


pectic substance that cements plant 
cell walls together. The result is a 
slimy, often foul-smelling mass. Tlie 
cell-wall-dissolving enzymes and tojdns 
often destroy cells and tissues a Aort 
distance ahead of the bacteria that pro- 
duce them. The soft rots often follow 
and extend invasion and damage by 
some other pathogen. For example, 
black rot of cabbage and late blight of 
potatoes would be much less serious 
except for the subsequent soft rot 

Symptoms of disease appear at vary- 
ing lengths of time after tocteria attack 
and grow in a plant. Soft rots arc some- 
times evident within a day or so, 
angular leaf spot of cotton within lo 
days, com wilt within i to 2 months. 
Grown gall of orange may take 2 years. 
The time between first establishment 
of bacteria or fungi in the plant and 
the appearance of symptoms of disease 
is call^ the incubation period. 

In some diseases bacterial ooze comes 
to the surface of affected plant parts. 
The exudate is often idimy and sticky 
and contains numberless bacteria. The 
ooze may come out of stomata or other 
natural openings, or may form on the 
surface of cankers or other lesions. 

We recognize three major variables 
that influence the eventual severity of 
an outbreak of disease — the host plant, 
the pathogen, and environmental con- 
didons. They form an eternal triangle, 
each affeedng the other within certain 
limits. 

The variables among the host plants 
are substantial and important. They 
make possible selection and breeding 
for disease resistance. The origin 
of disease-resistant breeding material, 
plant structures that affect resistance, 
and control of disease through the use 
of disease-resistant plants are discussed 
in detail in other ardcles in the Year- 
book. All ages of plants arc affected 
from the seedling stage through to ma- 
turity, Fruits and seeds may be at- 
tacked. Juiciness of tissue, however, 
may predispose a plant to severe at- 
tacks by bacteria. 

The relationship of the pathogenic' 



ftACTf ilA-^SMAlL AND MIOHTY I3 


bacteria to the hxm plant involves a 
chain of events, each link of which may 
be critical in determining the severity 
of an epidehiic. Among the more im- 
portant parts of this chain are the en- 
trance of the bacteria into the host 
plant, their establishment within the 
plant tissues, their exit again to the 
surface of the plant, and their distri- 
bution to another plant. This chain of 
events may be repeated over and over 
again during a growing season. More- 
over, the bacteria must be able to over- 
winter successfully if the epidemic is to 
develop the next year. Overwintering 
may be accomplished in the seed (bean 
blight, cabbage black rot); in tissue of 
perennial plants (fire blight of pear 
and apple, crown gall of brambles, 
roses, and many other kinds of plants) ; 
plant refuse (tomato wilt, potato ring 
rot); or the soil (crown gall, tomato 
wilt). 

An understanding of the cyclic chain 
of events necessary to produce a severe 
outbreak of disease often enables us to 
select its weakest link and suggests 
ways of breaking the infection cycle 
and thus of controlling a disease. Be- 
cause infected seed carry the organisms 
causing bean blight and cabbage black 
rot from one crop year to the next, we 
can use disease-free seed to break the 
chain and avoid heavy losses from 
those diseases. The bacteria that cause 
cucumber wilt are carried from plant 
to plant by cucumber beetles, and so 
control measures for the wilt include 
spraying to control the beetles. Losses 
from other diseases can be reduced 
through treatment of seed, selection of 
disease-resistant stock, and crop rota- 
tion. More effective, less exp>ensive, 
and more easily applied control meas- 
ures are needed for many diseases, 
however. 

A later article explains how the vari- 
ability of fungi affects the control of 
plant diseases. We know much less 
about variability in the plant disease 
bacteria. The vigorous pathogens ap- 
rarently survive in largest numbers. 
This continual selection makes patho- 
genicity — the abUity to cause disease — 


possibly the most stable characteristic 
in nature. 

Bacterial diseases of plants occur 
in almost every place that is reasonably 
moist or warm. Their destructiveness 
varies from year to year and place to 
place. A part of this variation can be 
attributed to presence or absence of a 
critical environmental condition under 
which the bacteria operate on the host 
plant. 

Along with temperature, moisture is 
extremely important. Abundant water 
in the soil and high relative humidity 
in the air encourage the plants to take 
in as much liquid as possible. The 
leaves are usually covered easily with 
water and any chemicals that might 
resist bacterial action are diluted. This 
often predisposes the cells to patho- 
genic invasion. Likewise, under those 
conditions, the stomata of the leaves 
may be wide open. Many of the patho- 
genic bacteria are spread from one 
plant to another by the distribution of 
splashing rain and running water. In 
fact, under the influence of beating 
rain, leaves may become partly water- 
soaked. Then most any soil bacteri- 
um — particularly a pathogenic form — 
is able to grow and to produce a ne- 
crotic lesion. Conversely, carrots and 
potatoes, for example, that have dried 
properly after digging are quite re- 
sistant to soft rot. 

Tem]:>erature has an especially im- 
portant part in the development of 
most bacterial diseases. This is seen in 
the tomato wilt disease, which may 
be present in various plantings but 
may not become epidemic until the 
warm weather raises the temperature 
of the soil. Bean blight is also a high- 
temperature disease. Infection, how- 
ever, may actually take^^ place over a 
wide range of temperatures, but the 
high temperature is needed for typical 
symptoms. Temperature is likewise a 
critical item in the development or 
lack of development of crown gall. 
Tomato plants grown at 89^ F. failed 
to develop galls while those at 89^ 
produced typically large cells. Air and 



14 YIAirftOOK OF AOftICULTURI 1953 


soil temperatures are important for 
the tomato canker disease which de- 
velops best at a soil temperature of 
82^. Cold storage effectively inhibits 
the bacterial rots that develop so 
rapidly with suitable warmth. 

Mineral nutrition may influence 
disease development. Stewart’s wilt 
disease of corn increases as the nitro- 
gen level increases, within limits. At 
high levels of nitrogen, infected plants 
die in a few weeks. Virulent strains of 
this bacterium grow well on inorganic 
nitrogen, while avirulent strains re- 
quire organic nitrogen. Weakly and 
highly virulent strains are equally de- 
structive in nitrogen-deficient plants, 
but highly virulent strains are much 
more so in plants that receive liberal 
amounts of nitrogen. The tomato wilt 
disease is affected strikingly by the 
nutrient level of the plants. 

The number of hours of sunlight 
during any season of the year ^so 
influences disease development. In the 
greenhouse less tomato wilt develops 
during an 18-hour day (simulating 
summer conditions) than under a 
12-hour day (winter or spring condi- 
tions). 

Nutrition, length of daylight, and 
temperature are all closely associated 
in the development of disease condi- 
tions. It is hard to assess the intensity 
of the effects of each taken alone. The 
three conditions influence the balance 
of inorgance and organic nitrogen and 
the carbohydrate supply in the plant 
sap and thereby favor or inhibit the 
pathogenic bacteria, depending on 
their requirements. 

In si*udies of bacterial plant dis- 
eases, the scientist must identify the 
organism causing the disease and be 
certain that his cultures have not be- 
come contaminated with an unwanted 
species. Studies under the microscope 
of properly prepared samples of the 
growth developing in cultures and of 
bacterial ooze or tissue from diseased 
plants furnish some evidence. 

Because the simple cells of the 
-pathogenic bacteria are so similar in 


appearance, however, additional tests 
of the effects of their action on various 
substances must be used to identify 
them. Their type of growth and col- 
ony formation on various semisolid or 
liquid culture media in petri plates 
or test tubes, their ability to ferment 
selected nutrient substances or to pro- 
duce acids or gas on them, and their 
ability to withstand more or less un- 
favorable physical conditions (such as 
high and low temperatures or certain 
chemicals) are among the reactions 
that are commonly used in identifica- 
tion. Their ability to cause plant dis- 
eases is the factor chiefly responsible 
for our present interest in them. That 
factor also is an aid in identifying an 
unknown organism. 

To DETERMINE thc identity of bac- 
teria, one has to have laboratory 
equipment for making culture media 
and maintaining cultures, for micro- 
scopic examinations, and for various 
physiological determinations. 

Suitable precautions and special 
techniques also are necessary to get 
and maintain cultures and media that 
are free from contaminations. 

The Committee on Bacteriological 
Technique of thc Society of American 
Bacteriologists, Biotech Publications, 
Geneva, N. Y., publishes a series of 
frequently revised leaflets about the 
latest techniques. Most up-to-date 
bacteriologists follow Bergey’s Manual 
of Determinative Bacteriology^ published 
by Williams and Wilkins, Baltimore, 
Md., as the standard textbook on 
identification. Thc sixth edition of this 
book was published in 1948. In that 
manual the plant pathogenic bacteria 
arc classified into seven genera — Agro» 
bacterium^ Bacterium^ Corynebacterium^ 
Erwiniay Pseudomonas^ Streptomyces^ and 
Xanthomonas. 

Charlotte Elliott, a former associate 
of Dr. Erwin F. Smith in the Depart- 
ment of Agriculture, wrote a book, 
Manual of Bacterial Plant Pathogens, The 
second ^ition, published in 1951 by 
the Chronica Botanica Company, of 
Waltham, Mass., gives the names and 



VIRUSiS. A SCOUROi OF MANKIND 


15 


descriptions of plant pathogenic bac- 
teria, their redactions on culture media, 
symptoms of the diseases they produce, 
their host plants, geographical distribu- 
tion, control methods if known, and 
citations to research literature. 

How DOES a plant pathologist make 
sure that a plant disease is caused by a 
given species of bacterium? Robert 
Koch in 1882 worked out the rules of 
proof to follow for animal diseases. 
The rigid logic of his requirements 
applies equally well to bacteria causing 
plant disease. 

Briefly stated, the {postulates of Koch 
require that: (i) The bacterium must 
be associated in every case with the 
disease, and conversely the disease 
must not appear without it. (2) The 
micro-organism must be isolated in 
pure culture and its specific morpho- 
logical and physiological characteris- 
tics determined. (3) When the host is 
inoculated with the bacterium under 
favorable conditions, the characteris- 
tic symp^ms of the disease must 
develop. (4) The micro-organism must 
be reisolated from the inoculated 
plant and identified as that first 
isolated from the diseased host. 

In this brief review we have not 
attempted to present detailed informa- 
tion on individual diseases. Losses 
caused by various important bacterial 
diseases, symptoms, means of spread 
and control measures when known are 
given in other articles on specific crops. 

A. J. Riker is a member of the depart^ 
merit of plant pathology^ University of 
Wisconsin. He has made many contribu^ 
tions to our knowledge of bacterial diseases 
of plants and has conducted research on the 
basic reasons for diseased growth. Dr. 
Riker is alio an authority on the diseases 
of forest trees. 

A. C. Hildebrandt, also of the 
department of plant pathology^ University 
of Wisconsin^ conducts research on funda- 
mental phases of bacterial diseases of plants 
with special emphasis on factors affecting 
the growth of organisms and plant tissue in 
culture media. 


Viruses, a 
Scourge of 
Mankind 

C. W. Bennett 

Few kinds of living organisms are 
immune to viruses. Man, domesticated 
and wild animals, insects, plants, and 
even bacteria succumb to their attack. 
They have been a scourge of mankind 
since before the dawn of recorded 
history. Smallpox, for example, ex- 
isted in China in 1 700 B. C. Measles, 
mumps, influenza, and scarlet fever 
are other virus diseases that plague 
humans. 

The virus diseases of plants also 
probably have existed for many cen- 
turies. Their importance on crop 
plants has increased tremendously in 
the past 50 years — since 1900 more 
than 200 new plant viruses have been 
discovered. Many of them have done 
widespread damage to crop plants. 

Curly top caused almost complete 
abandonment of the sugar beet indus- 
try in parts of western United States 
from 1916 to 1932 and still causes 
severe injury to tomatoes, beans, and 
a number of other crops. Sugarcane 
mosaic caused extensive losses to the 
sugarcane industry in the United 
States, Argentina, Brazil, and other 
countries beginning about 1917. Spot- 
ted wilt has become widespread and 
now causes losses to tomato and other 
crops in many parts of the world. Since 
1940, swollen shoot has caused ex- 
tensive damage to the cacao industry 
of west Africa. Virus diseases of citrus 
have become more destructive and 
from 1936 to 1946 tristeza catised the 
loss of 7 million orange trees in the 



YEARBOOK OF AGRICULTURE 1953 


l6 

State of Sao PauJo, Brazil, alone. It 
has attacked or now threatens other 
millions of trees in various tropical and 
subtropical areas. 

This increase in destructiveness of 
virus diseases and in the number of 
known viruses has come about largely 
as a result of the expansion of agri- 
cultural enterprises and the increased 
movement of plants and plant prod- 
ucts in recent years. 

Disease-producing agents — particu- 
larly viruses — apparently have origi- 
nated in local areas all over the world. 
Probably through long association 
have native plants developed a toler- 
ance to the local viruses that enables 
infected individuals to survive with 
little injury and often with but little 
evidence of being carriers of viruses. 

When crop plants are introduced 
into areas where they have not been 
grown before, they frequently become 
subject to infection with the native 
viruses, against which they have had 
no opportunity to develop resistance. 
Such a virus may cause extensive 
losses to a crop plant, not only in the 
areas of original distribution of the 
virus, but also in the other areas to 
which it may spread on the recently 
attacked crop plant. 

The appearance of the yellow wilt 
virus on sugar beet in the Rio Negro 
Valley of Argentina w^hen attempts 
were made to develop a sugar beet 
industry mere in 1929 probably is a 
typical example of the transfer of a 
virus from wild plants to a new crop 
plant. From the beginning of the 
attempt to establish the sugar beet 
industry there, the sugar beets were 
attacked by a virus disease, previously 
unknown on sugar beet or on any 
other plant. It caused a yellowing, 
wilting, and death of all infected 
plants. Injury was so severe that 
attempts to grow sugar beets were 
abandoned about 10 years later after 
considerable loss to growers and proc- 
essors. This disease appears still to be 
limited to South America, but it con- 
tinues to constitute a potential threat 
to sugar beets elsewhere. 


Many plant viruses have spread over 
extensive areas from their points of 
origin. The movement of the tristeza 
virus of citrus is an example of the 
extensive spread of a destructive virus 
disease. Tristeza virus probably origi- 
nated in South Africa or in south- 
western Asia. For many years, possibly 
for centuries, it appears to have had a 
limited distribution but since 1930 it 
(or a similar malady or maladies) has 
been reported in Australia, Java, 
California, Louisiana, Argentina, Uru- 
guay, and Brazil. Apparently the chief 
method of long-distance spread has 
been by means of infected budwood 
or nursery stock moved from infested 
to noninfested areas. After it was in- 
troduced into Brazil and Argentina it 
was spread rapidly by the oriental 
citrus aphid, Aphis citricidus. 

Virus diseases produce a wide range 
of symj)toms and types of injury on 
plants. Sometimes they kill the plant 
in a short time, as with spotted wilt 
and curly top on tomato. More often 
they cause lesser injuries that result 
in reduced yields and lower quality 
of product. With respect to general 
type of the symptoms produced, most 
viruses are of two rather clearly de- 
fined groups: Those that cause mot- 
tling. or spotting of leaves and those 
that cause a yellowing, leaf curling, 
dwarfing, or excessive branching, but 
little or no mottling or spotting. 

I'hc first group, by far the larger, 
includes such important viruses as 
those causing cucumber mosaic, peach 
mosaic, and the tomato spotted wilt. 
Mosaic diseases arc characterized by 
the production of chlorotic, or yellow- 
ish, areas in the leaves and sometimes 
in the blossoms and other parts The 
chlorotic areas may be more or less 
circular or they may be irregular. The 
spots vary in size from very small areas 
to large blotches. Sometimes the chlo- 
rotic areas cover a large share, or even 
all, of the surface of individual leaves. 

Some of the mosaic viruses cause 
conspicuous mottling, spotting, or 
striping of petals of flowers of some 
ornamental plants. Many of the varie- 



VIRUSES, A SCOURGE OF MANKIND 


gated tulips are only virus-infected 
plants of nonvariegated varieties. The 
Rembrandt variety is a virus-infected 
strain of the variety Princess Eliza- 
beth. Variegation, or ‘'breaking,” of 
blossoms due to viruses commonly 
occurs also in wallflower, slock, gladi- 
olus, and flowering peach. 

The attractively variegated leaves 
of the ornamental shrub Abutilon 
striatum are also the result of a virus 
infection. In the United States the 
causal virus apparently has no insect 
vector and variegated and nonvarie- 
gated plants of A. striatum grow side by 
side indefinitely with no spread of 
the variegated condition to the non- 
variegated plants. In Brazil the virus 
is transmitted by the whitefly, Bernisia 
iahaci^ and there healthy plants may 
soon become variegated. 

Other mosaic-type viruses cause 
circular or irregular necrotic — dying — 
or chlorotic spots on leaves, sterns, and 
fruits, as in spotted wilt on tomato. 
Ring spots and oak-leaf patterns are 
symptoms of other virus diseases. 

Some viruses produce all these types 
of symptoms and others as well under 
specific conditions or on difl'erent host 
plants. 

Diseases of the leaf curl and yellows 
group are caused by viruses that ap- 
pear to l3e associated wdth the vascular 
system of the plant and produce symp- 
toms that arc characteristic of disturb- 
ances in this type of tissue. With the 
leaf curl diseases, such as raspberry 
leaf curl, sugar beet curly top, and 
tobacco leaf curl, grow'th of veins is 
retarded and the leaves roll or become 
crinkled. Leaves are sometimes deeper 
green than normal. The yellow's types, 
such as aster yellow's and sugar l>eet 
yellow wilt, cause yellowing, stunting, 
and various types of leaf deformations. 
Other diseases, .such as strawberry 
stunt, cause dwarfing of the plant and 
some leaf rolling and thickening. 
Still others stimulate the production 
of clusters of thin, wiry shoots as in 
witches’-broom of oceanspra>’, or they 
cause the production of wiry shoots on 
stems or on main limbs and trunks if 


n 

trees arc attacked. Viruses of the leaf 
curl and yellows group seem to persist 
in their insect vectors for longer 
periods than do viruses of the mosaic 
group. 

Some \'inises produce symptoms only 
under certain environmental conditions 
or on certain host plants or host com- 
binations. Red raspberry mosaic causes 
mottling only on leaves produced at 
low' temperatures. Symptoms usually 
appear therefore only on leaves formed 
in early spring or late fall. Curly top 
virms is present in many plants of tree 
tobacco, Nicotiana glauca^ in California, 
but no symptoms are evident. Tristeza 
virus produces marked symptoms only 
on citrus trees of certain top and root 
combinations. For example, both sweet 
and sour orange trees grow' well even 
thougli infected w'ith the tristeza virus. 
Also, trees of sour orange that arc 
grafted onto sw'cet orange rootstocks 
show little evidence of injury. How- 
ever, if the positions of the two types of 
orange are reversed in the grafted tree 
so that the top of the tree is of sweet 
orange and the rootstock is of sour or- 
ange, the tristeza virus causes yellow- 
ing and dropping of the leaves, grad- 
ual dying back of tw'igs, production of 
weak shoots on the main limbs, and 
eventual death of the tree. 

Plants may be infected w'ith two or 
more ' iru.ses at the same time. When 
a plant with one viru.s disea.se is at- 
tacked by another, usually symptoms 
of the .second disea.se are merely super- 
imposed on those produced by the first. 
Occasionally, however, infection by 
two viruses results in added symptoms. 
An example is double-virus streak in 
tomato cau.sed by infection with both 
tobacco mosaic virus and potato virus 
X. Tobacco mosaic virus causes mot- 
tling and a certain ari\ount of dwarf- 
ing. Potato virus X induces mild mot- 
tling. When both viruses are present in 
the same tomato plant a marked in- 
crease in injury occurs w'ith the produc- 
tion of extensive necrosis on leaves and 
stems not characteristic of either virus 
alone. 

Transmission of virus diseases may be 



YEAIBOOK OP AGtICULTURB 1953 


l8 

brought about in a number of different 
ways. Under experimental conditions, 
many of the mosaic-type viruses can be 
transmitted mechanically by rubbing 
juice from diseased plants over the sur- 
face of leaves of healthy plants. The ad- 
dition of a small amount of an abrasive 
such as carborundum to the juice fre- 
quently increases the amount of infec- 
tion. This method of transmission is of 
great value in studies of properties, 
characteristics, and host range of vi- 
ruses. The majority of viruses, how- 
ever, have not been transmitted by 
juice inoculation. 

Transmission of sugar beet curly top 
virus and cucumber mosaic virus by 
means of dodder, Cuscuta subinclusa, 
was reported in 1 940, and a number of 
other viruses have been transmitted 
since by it and other species. 

The species of dodder used for virus 
transmission are members of a group 
of interesting parasitic seed plants be- 
longing to the same family as morning- 
glory and sweetpotato. They some- 
times cause extensive injury by para- 
sitizing clover, alfalfa, and other plants. 
They attach themselves to their host 
plants by a type of natural graft union. 
Because species of dodder have exten- 
sive host ranges, plants widely different 
botanically may be united by a type of 
natural graft in which dodder forms a 
connecting link through which several 
viruses have been found to pass. Thus 
dodder provides a medium through 
which viruses may be transferred from 
diseased to healthy species that cannot 
be infected by juice inoculation or by 
insect vectors. 

Under field conditions only a few vi- 
ruses arc disseminated by mechanical 
transfer. Most natural spread takes 
place through the use of infected vege- 
tative parts for propagation, through 
infected seeds, and by insects. 

The virus of tobacco mosaic prob- 
ably is the only plant virus spread 
extensively and chiefly by mechanical 
means. It occurs in such high concen- 
trations in infected plants, persists for 
such long periods in infested plant 
material, and is so easily spread by 


contact that extensive transmission in 
tobacco may occur in operations in- 
volving handling plants at planting 
time and in the cultural operations 
throughout the growing season. Also, 
it is spread in greenhouse tomatoes 
when they are cultivated, pruned, tied 
to supports, or harvested. 

Propagation of plants by vegetative 
means results in the spread of many 
virus diseases. Most viruses invade 
plants so extensively that they occur 
in all parts of the plant, and many of 
them undoubtedly invade nearly all of 
the living cells. When buds or scions 
from virus-infected plants are used for 
propagation, the new plants arc nearly 
always infected. Likewise, the tubers, 
roots, offshoots, or other vegetative 
parts used for propagation carry all of 
the viruses present in the parent plant. 
For this reason transmission by vege- 
tative propagation is important in the 
dissemination of vii-us diseases of straw- 
berry, raspberry, potato, tree fruits, 
and many ornamentals. Most of the 
viruses of those plants are spread by 
other agencies as well. 

Seed transmission of viruses occurs 
only in relatively few cases. Seeds, for 
the most part, have a remarkably effec- 
tive mechanism that prevents the pas- 
sage of virus into the young embryo 
from the mother plant. The nature of 
this mechanism is imperfectly under- 
stood, but its effectiveness may be 
illustrated by the reaction of sugar beet 
to curly top virus. The curly top virus 
is known to spread throughout the 
plant, and seeds of diseased plants 
carry high concentrations of virus. 
Despite this fact, however, the virus 
docs not enter the embryo and the 
infected seeds always produce virus- 
free plants. 

Even the relatively few viruses 
known to be seed-borne, such as bean 
mosaic virus, cucumber mosaic virus, 
lettuce mosaic virus, peach ring spot 
virus, and dodder latent mosaic virus, 
are carried only in a portion of the 
seeds of diseased plants. In most in- 
stances, in fact, only a low percentage 
of such seeds produce infected plants. 



VIRUSIS, A SCOURGE OF MANKIND 


Transmission in seed, however, may 
be important, particularly with dis- 
eases like lettuce mosaic, a few diseased 
seedlings of which may be the source 
for extensive spread by insects. 

Some of the seed-borne viruses are 
carried in the pollen. When healthy 
bean plants are fertilized by pollen 
from mosaic plants, a certain propor- 
tion of seeds from the healthy plants 
produce diseased seedlings. This, how- 
ever, appears not to be an important 
method of virus spread. 

Insects are by far the most important 
agents of transmission. Few viruses 
could persist for long without them. 

Properties and characteristics of 
plant viruses have been subjects of 
study and speculation for many years. 
Information has increased with more 
extensive investigations and with the 
development of new techniques and 
instruments. 

It was learned more than 50 years 
ago that viruses are very minute entities 
capable of passing filters so fine that 
they retained common forms of bac- 
teria and that viruses apparently do 
not increase outside of living cells. 

Further studies made in plants and 
with plant juices revealed that viruses 
vary widely in their reactions to a num- 
ber of environmental conditions. Many 
appear to be inactivated almost imme- 
diately in juice after it is pressed from 
diseased plants. This probably accounts 
in part for the fact that many viruses 
cannot be transmitted by juice inocu- 
lation. Other viruses retain activity in 
plant juice for different periods. Cu- 
cumber mosaic virus remains active 
for only a few hours, sugar beet mosaic 
virus for i to 2 days, and curly top 
virus for 2 to 5 days. Tobacco mosaic 
virus is much more resistant, however, 
and may retain its activity in juice 
from diseased tobacco plants for several 
months. 

Some viruses that can be transmitted 
by the juice cannot be recovered from 
diseased plants after they are dried. 
Others retain activity in dried plants 
longer than in plant juice. Curly top 


^9 

virus was recovered from dry sugar 
beet plants after 8 years, and tobacco 
mosaic virus has retained its activity in 
dried tobacco leaves for more than 50 
years. 

Peach yellows virus is sensitive to 
high temperatures and can be inacti- 
vated in young peach trees by growing 
them at a temperature of 95^ F. for 3 
weeks or by immersing dormant trees 
in water at 122® for 10 minutes. It is 
destroyed by summer temperatures 
that prevail for extended periods in 
some parts of the United States. Aster 
yellows virus also is inactivated at rela- 
tively low temperatures, and its spread 
is reported to be more extensive in the 
spring and fall than during the summer 
because of inactivation of the virus in 
the leafhopper vector during periods 
of high temperatures. Most viruses, 
however, have thermal inactivation 
points well above the temperatures 
required to kill the host plants. Some 
viruses are very resistant to heat. Sugar 
beet curly top virus is inactivated 
betw'cen 167® and 176®, and tobacco 
mosaic virus is not inactivated until 
the temperature rises to 192®. 

As a rule, the viruses resist the action 
of the common germicides. Usually 
they are not inactivated by the con- 
centrations of bichloride of mercury, 
carbolic acid, copper, formaldehyde, 
and alcohol that kill most bacteria. 
Curiously enough, tobacco mosaic 
virus, in other respects one of the most 
stable of viruses, is easily inactivated 
with alcohol. 

The isolation and purification of the 
tobacco mosaic virus by Dr. W. M. 
Stanley in 1932 opened the way for 
more direct and more comprehensive 
studies of the nature and character- 
istics of viruses than had been possible 
by use of earlier methods. The purifi- 
cation of tobacco mosaic virus was 
soon followed by purification of other 
plant viruses, some by chemical means 
and others by sedimentation in high- 
speed centrifuges. 

Most of the viruses that have been 
obtained in relatively pure condition 
have been found to aggregate to form 



20 


YEARBOOK OF AORICULTURB 1953 


crystals or crystal-like bodies that arc 
readily visible under low magnifica- 
tion. Some arc even large enough to 
be seen with the unaided eye. The 
crystals differ in size and shape de- 
pending on the virus and the conditions 
under which they are formed. Crystals 
of tobacco mosaic virus are needle- 
shaped; those of tobacco necrosis virus 
lozenge-shaped, and those of tomato 
bushy stunt virus are dodecahedral. 

Even after viruses were purified, the 
virus particles themselves could not 
be seen because their sizes arc below 
the limits of resolution of the ordinary 
microscope. That deficiency was over- 
come with the perfection of the elec- 
tron microscope, which utilizes radia- 
tions of shorter wavelength than those 
of light. This microscope permitted 
particles in the size range of \ iruses 
to be photographed. For the first time 
it was possible to determine what 
viruses look like in photographs, even 
if not actually to sec them. 

The plant viruses that have been 
photographed by means of the electron 
microscope, numbering a dozen or 
more, extend over a consideraljle 
range in size and shape. Some are 
long, straight, or tenuous rods. Some 
are shorter rods. Others arc spherical. 
Tobacco mosaic virus particles arc 
relatively straight rods about 1 5 milli- 
microns in thickness and varying in 
length up to 300 millimicrons, or 
more. (One millimicron is one-mil- 
lion th of a millimeter or about one 
twenty-five-millionth of an inch.) The 
potato virus X also has rodlike parti- 
cles, which vary greatly in length and 
appear to be more flexible than the 
tobacco mosaic virus particles. Among 
the viruses that appear to have spher- 
ical particles are alfalfa mosaic virus, 
With a diameter of about 17 millimi- 
crons; tobacco ring spot virus, with a 
diameter of about 19 millimicrons; 
and bushy stunt virus, with a diameter 
of about 26 millimicrons. 

The smallest plant viruses are some- 
what smaller than some protein mole- 
cules. The largest ones are smaller 
than the smallest known bacteria. 


Thus, viruses as a group cover a size 
range between chemical molecules 
and bacteria, with some overlapping 
in the lower size range. 

All plant viruses so far isolated are 
similar in chemical composition and 
contain two essential constituents, 
nucleic acid and protein. 

Nearly all of the plant viruses that 
have been studied extensively have 
been found to be complexes of strains. 
The strains vary in virulence, kind of 
disease produced, host range, or other 
characteristics. Curly top virus and 
tobacco mosaic virus are made up of 
innumerable strains. Some of these 
are weak and cause little injuiy even 
on susceptible plants. Others arc more 
virulent and cause severe injury. Some 
strains of curly top virus attack 
tobacco and tomato; others do not. 
Two strains of potato yellow dwarf 
virus appear to be transmitted by 
different specific vectors. 

It may be assumed that these strains 
have arisen, one by one, from parent 
strains during the course of years by a 
process similar to mutation. The rela- 
tively large numbers of strains of some 
of the more common viruses may indi- 
cate that some viru.ses are less stable 
than others, under the environmental 
conditions to which they arc subjected, 
hence mutate more readily. This tend- 
ency to mutate gives to viruses a degree 
of adaptability that may be compa- 
rable to that possessed by plants and 
animals. 

Recovery from the more severe 
phases of virus dLsease is a rather com- 
mon phenomenon in plants but is lim- 
ited usually to partial or complete 
recovery from obvious symptoms and 
very rarely extends to loss of the virus 
by infected plants. 

One of the first examples of recovery 
to be discovered was that of the recov- 
ery of tobacco from ring spot. A few 
days after plants become infected with 
this virus, marked necrosis is produced 
on a ring of new leaves, but subsequent 
growth is normal or nearly normal. 

Plants from cuttings of recovered 
plants remain free of severe symptoms^ 



21 


VIRUSIS, A SCOURGE OP MANKIND 


and the diseased plants may be prop- 
agated indefinitely by vegetative means 
without the reappearance of the severe 
phase of the disease. 

Turkish tobacco recovers to a very 
high degree from curly top. Tomato 
plants are killed by curly top if the 
virus is introduced by means of the beet 
leafhopper; if it is introduced by means 
of scions from recovered tobacco, how- 
ever, they usually show only mild 
symptoms. 

Water pimpernel, Samolus parviflorus, 
also shows a high degree of recovery 
from most strains of curly top virus. 
Recovery in sugar beet is much less 
marked. Plants of susceptible varieties 
usually show little evidence of improve- 
ment after they become diseased. 

With some viruses and perhaps with 
most of them, infection by one strain 
protects against infection or injury 
from a second strain of the same virus, 
but no protection is afforded against 
infection with a totally different virus. 
Protection of this type is very marked 
between strains of the tobacco mosaic 
virus. Juice from a plant with tobacco 
mosaic, w'hcn rubbed gently over the 
surface of a leaf of a healthy plant of 
Nicotiana sylvestris, produces many 
small necrotic lesions, which mark 
points of separate infections. If the 
inoculated plant has already been 
invaded by a strain of tobacco mosaic 
virus, howT.vcr, no lesions are produced 
by reinoculation with a second strain 
of the same virus. 

After recovery from one strain of 
curly top virus, Turkish tobacco plants 
arc very resistant to injury by other 
strains of this virus. Also, when tomato 
plants are inoculated with one strain of 
the curly top virus by means of scions 
from recovered tobacco plants, they 
are very resistant to injury by most 
other strains. Tomato plants “immu- 
nized” in this way have been grown 
successfully under field conditions 
where all nonprotected plants were 
killed. 

In contrast, the plants of Nicotiam 
glutinosa and water pimpernel, al- 
though they recover from the severe 


effects of most strains of the curly top 
virus, are not protected against injury 
by strains of the virus more virulent 
than the one already present. No strain 
of curly top virus offers appreciable 
protection against either irffection or 
injury in sugar beet. 

Although protection between related 
strains of some viruses is npt always 
evident or complete, cross-protection 
tests have been of value in identifica- 
tion and classification of viruses from 
different sources and host plants. 

The cross-protection phenomenon 
may prove to be of value also in certain 
phases of control of virus disease, par- 
ticularly diseases such as swollen shoot 
of cacao and tristeza of citrus. Where 
either is present, it is known that all 
susceptible trees must sooner or later 
become infected from virus sources that 
are impossible to eliminate. Since this 
is true, it may provc’worth while first 
to infect all the young planting stock 
with a weak and relatively harmless 
strain of virus in order to protect 
against later infection by more virulent 
strains of the same virus. 

After a plant is infected with a virus 
disease nothing, as a rule, can be done 
to restore its health. Therefore, meth- 
ods of control arc directed almost 
wholly toward prevention of infection 
or toward development of disease- 
resistant varieties. 

Many practices and precautions are 
employed to prevent infection with 
virus diseases. Control of mosaic on 
tobacco and tomato is obtained largely 
by avoiding spread of virus by con- 
tact during transplanting and cultural 
operations. Fortunately, few of the 
virus diseases arc so easily transmitted 
and ordinary cultural operations can 
be carried on with most plants without 
danger of spreading virus diseases by 
contact. 

Destruedon of plants that serve as 
sources of infection is of value in the 
control of a number of virus diseases. 
Spring infection of fields of sugar beets 
with mosaic and virus yellows comes 
largely from infected beets that sur- 
vive the winter or from beets that are 



122 


YEARBOOK OF AGRM:ULTURE 1953 


carried through the winter for seed 
production. Elimination of such sources 
of virus usually gives a high degree 
of control. The spread of X-disease on 
peach can be prevented by removing 
all infected chokecherries within 500 
feet of peach orchards, and the virus 
diseases of raspberries can be con- 
trolled, in most instances, by destroy- 
ing all wild and escaped brambles in 
the immediate vicinity of plantings, 
provided the plantings themselves are 
not already infected. 

Reducing the population of insect 
vectors by spraying or by other means 
has value in the control of some virus 
diseases. Usually it is not possible, 
however, to reduce the insect popu- 
lations sufficiently or soon enough to 
obtain completely satisfactory results. 
Some virus diseases can be partly con- 
trolled by destruction of the hosts of the 
insect vectors. Extensive reduction of 
the weed hosts of the beet leaf hopper 
in the Western States would corre- 
spondingly reduce the amount of curly 
top virus carried from desert plants to 
cultivated fields. In much of this area, 
reduction in weed hosts comes about 
naturally under systems of land man- 
agement in wh'ch annual and peren- 
nial grasses and other nonhost plants 
are allowed to replace the weed hosts 
of the beet leaf hopper. Fall spraying 
to kill leafhoppers on weeds in un- 
cultivated areas has been resorted to 
also in the program to control curly 
top. 

Virus-free nursery stock is extremely 
important in the control of virus 
diseases of strawberry and raspberry. 
Natural virus spread often is not ex- 
tensive enough to cau.se serious dam- 
age during the life of plantings started 
with virus-free nursery stock. That is 
true also of some of the virus diseases 
of tree fruits. 

C. W. Bennett is a pathologist of the 
division oj sugar plant investigations ^ 
Bureau oJ Plant Industry^ Soils, and Agri-- 
cultural Engineering, who has had more 
than JO years oJ experience in the study of 
plant viruses and virus diseases. 


How Insects 

Transmit 

Viruses 


L, M. Black 

Most viruses that cause plant disease 
are transmitted ..by insects, principally 
those that have sucking mouth parts — 
aphids, leafhoppers, white flies, mealy-- 
bugs, and tingids. Leafhoppers and 
aphids are the most important. 

Although many plant viruses are 
without knowm insect vectors, it is 
generally expected that insect carriers 
will eventually be discovered for most 
of them. There are exceptions. To- 
bacco mosaic virus and potato latent 
virus arc two virgses that occur in 
high concentration in infected plants 
and are stable enough to be spread 
readily from one plant to another by 
almost any means that releases juice 
from a wound in an infected plant and 
transfers the juice to a fresh wound in a 
healthy plant. Tobacco mosaic virus is 
thus tran.sferred by the hands of men 
working with tobacco plants even 
though the wounds may be only micro- 
scopic in size. It can also be thus trans- 
ferred by the mouth parts of grass- 
hoppers. Potato latent virus can be 
transferred from plant to plant when 
the wind blows the leaves of diseased 
plants against healthy ones so as to 
injure both. The mystery about these 
two viruses is not their transmission by 
such methods but why potato latent 
virus is apparently not transmitted by 
sucking insects and why tobacco 
mosaic virus is so poorly transmitted. 

A few viruses, such as wheat rosette 
virus and lettuce big-vein virus, con- 
taminate the soil in which diseased 



HOW INSECTS TRANSMIT VIRUSES 


plants are grown and infect healthy 
plants sub^quently grown therein. 
Just how inoculation takes place in 
such diseases is not known. Dodders 
{Cuscuta species), parasitic flowering 
plants, can transmit plant viruses by 
means of the natural graft unions they 
make with their hosts. Most plant vi- 
ruses, however, depend on insects for 
their dispersal. 

Aphids transmit more plant viruses 
than any other group. Aphid-borne 
viruses induce in plants a great variety 
of symptoms, the most important of 
which arc the mosaics. One of the 
most efficient aphid vectors is the green 
peach aphid, Myzus persicae^ which 
transmits more than 50 different plant 
viruses. 

Much remains to be learned about 
what actually occurs during trans- 
mission by aphids. Many aphid vec- 
tors transmit virus after very brief 
feedings on diseased plants. Studies on 
this type of transmission have reached 
a point where the feeding intervals of 
individual aphids are closely observed 
and timed by a stop watch. For 
example, the vector of beet mosaic 
virus requires an acquisition feeding 
of only 6 to 10 seconds. During subse- 
quent conset utivc inoculation feedings 
of 10 seconds each on healthy plants, 
the virus is gradually lost by the aphids 
and fewer than 2 percent of them can 
transmit to more than four plants 
without fresh access to virus. This virus 
is said to be nonpersistent in the vector. 
Usually such a virus is lost more 
rapidly from feeding aphids than from 
fasting ones — the virus of cucumber 
mosaic disease, for instance, is lost by 
the aphid within 6 to 8 hours when 
fasting, but within 10 to 20 minutes 
when feeding on healthy plants. In 
other instances this relation may be 
reversed. The loss of virus from aphids 
during feeding may be due to a virus- 
inactivating enzyme secreted by the 
aphids while feeding but not while 
fasting. Such a substance has not been 
deihonstrated, however, and in reality 
we do not know the explanation for 


23 

such loss and for many other features 
of aphid transmission. A virus trans- 
mitted by aphids, particularly if it is 
of this nonpersistent type, is usually 
transmissible by several or, indeed, 
many species. The virus of onion yel- 
low dwarf can be transmitted by more 
than 50 sp>ecies of aphids, but not by 
thrips, mites, grasshoppers, beetles, 
caterpillars, or maggots. 

That kind of transmission is in con- 
trast to another type in w^hich, follow- 
ing acquisition of virus, a latent period 
must elapse before the aphid is able 
to transmit. The aphid may then do 
so for many days without fresh access 
to virus. A minority of viruses trans- 
mitted by aphids arc spread in that 
manner. One of them, the virus of po- 
tato leaf roll, is not transmitted by 
the aphid until 24 to 48 hours have 
elapsed after acquisition. The virus 
may then be retained by the insect for 
7 to 10 days, even through molts, with- 
out fresh access to virus from plants. 

That an aphid may transmit one 
virus in the persistent manner and 
another in the nonpersistent from the 
same host plant clearly indicates that 
persistence or nonpersistence is de- 
termined by the^ virus. 

Why do we concern ourselves with 
such minute details of transmLssion? 
Simply because such knowledge may 
make the difference between success 
and failure in finding a vector of a 
virus. For instance, no transmission 
of a certain mosaic virus could be ob- 
tained after extensive trials with three 
species of aphids when they were fed 
one day on diseased plants and then 
transferred to healthy ones. When the 
aphids were fasted for 30 minutes be- 
fore an acquisition feeding of 5 to 10 
minutes and were then allowed an 
inoculation feeding of 5 to 10 minutes, 
however, transmissions were obtained. 

Leafhoppers, next to aphids, are 
the most important vectors of plant 
viruses. Experiments by a Japanese 
grower in 1884 demonstrated a con- 
nection between rice stunt and leaf- 
hoppers. That might be considered 



YEARBOOK OF AGRICULTURE 1953 


24 

the first virus shown to be insect- 
transmitted, but actually it was not 
realized until more than 20 years later 
that the causal agent of the disease 
was not the leafhoppcr but some au- 
tonomous agent carried by the insect. 

Viruses transmitted by leafhoppers 
cause a variety of symptoms in plants, 
including chlorotic streaking of leaves 
(as in corn streak), necrosis or death of 
tissues (as in elm phloem necrosis), 
tumors (as in wound-tumor disease), 
and yellows (as in aster yellows). All 
known vectors for virus diseases with 
a symptom picture like that of aster 
yellows aie leafhoppers. 

Although many aphid-bome viruses 
can be transmitted by rubbing leaves 
with juice from diseased plants, only 
two leafhopper-borne viruses have 
been so transferred. In other cases 
transmission has been accomplished 
only by the use of insects, dodder, or 
grafting. This has accordingly made 
the study of the viruses themselves 
very difficult. For such researches it 
has been necessary to permit the leaf- 
hoppera to feed on virus solutions 
through membranes or to inject the 
virus solution into leafhoppers. The 
insects must then be tested for infec- 
tivity on plants because none of the 
leafhoppers themsoLves has ever been 
observed to Ijc diseased. 

Practically all of the leafhopper- 
borne viruses (alfalfa dwarf virus is 
apparently an exception) arc con- 
sidered to have a latent or incubation 
period in their vectors. In some, this 
latent period may be so short (curly 
top virus) as to suggest that the virus 
does not multiply in the insect. Never- 
theless, it reaches relatively high con- 
centrations and is retained for weeks, 
not only in the beet leafhopper, which 
transmits it, but also in a number of 
other arthropods that cannot do so. 

In most leafhoppcr vectors that have 
been studied, however, the period that 
occurs between acquisition of the virus 
and its transmksibility by the vector 
is much longer. In many it varies from 
I to 2 weeks or more. This is a true 
incubation period, during which the 


viruses multiply to an infective con- 
centration in their vectors. 

Although most leafhopper vectors do 
not transmit virus to their progeny 
through the egg, certain exceptions 
exist. Rice stunt virus and clover club 
leaf virus may be passed to 95 or too 
percent of young insects through the 
eggs of the vectors. A single female 
leafhopper carrying clover club leaf 
virus has originated at least 2 1 genera- 
tions of infective progeny during a 
5-year period without fresh access to 
virus from plants. The virus in the 
original female had been diluted 
at least 100,000,000,000,000,000,- 
000,000,000 times. That would be 
impossible had not the virus multi- 
plied in the leafhopper. 

There may be, then, two main types 
of transmission of virus by leafhoppers. 
One type, exemplified by curly top 
virus, may be characterized by a very 
short incubation period and no multi- 
plication in the vector; the other type 
by a long period of incubation and 
multiplication in the vector. 

Once infective, lcafhopf>ers tend to 
remain so for many days without fresh 
access to virus, often until they die. 
Nevertheless, such insects may fail to 
infect susceptible plants for many days 
in succession. Some that obtain virus 
from their parent through the egg may 
in turn pass virus to their progeny 
through the egg and yet may fail to 
infect any susceptible plants although 
fed on them for their entire life. 

Often scientists have tested so many 
species of insects before finding a leaf- 
hopper vector that when they attained 
suceps they regarded the vector as 
specific. Only one species is known 
even today to transmit North Ameri- 
can curly top virus. Very likely that is 
because it is the only species of the 
genus that occurs in North America 
where tests have been made. 

The concept of specificity which en- 
visioned a single leafhopper species as 
the vector of a virus has been broken 
down by recent research. For example, 
it is now known that Pierce’s disease 
of grapes is transmitted by 24 different 



HOW INSECTS TIANSMIT VIRUSES 


species of leafhoppers in two families. 
On the other hand, work with yellow 
dwarf and with curly top viruses has 
revealed a new type of specificity, of 
considerable complexity. In the for- 
mer, there are two varieties of virus, 
each specifically transmitted by related 
leafhoppers. In the case of curly top, 
we have evidence of a complex of 
related viruses, with different vector 
and plant-host relationships. 

Much of the wide variation observed 
in vector efficiencies of individuals 
within a leafhopper species may be 
genetic. For example, ability to trans- 
mit com streak virus can be deter- 
mined by a single sex-linked dominant 
gene in the insect vector. In the case 
of the leafhopper that carries New 
York potato yellow dwarf, multiple 
factors are involved, the virus having 
been transmitted by 8o percent of the 
“active” insects, 2 percent of the “in- 
actives,” and 30 percent of hybrids. 

Considerable experimental work has 
been done on various asf^ects of the 
transmission process by the leafhopper. 
The mechanics of the mouth parts, the 
tissues of the plants reached by the 
mouth parts, and the location of the 
virus within the insect vector have all 
been subjects of investigation. Most 
leafhoppers apparently acquire virus 
from and ind’oduce vims into the 
phloem. However, some vectors feed 
on the xylem and acquire virus from and 
inoculate it into this tissue. Some vi- 
ruses show a corresponding specializa- 
tion in regard to the tissues they 
attack. 

White flies transmit a number of 
plant viruses. Nymphs of the white fly 
are attached to the plant and, therefore 
cannot themselves spread vims, but 
virus can be acquired by the nymphs, 
can pass through the pupal stage, and 
can be transmitted by the adults. The 
adults also can acquire the vims di- 
rectly from plants. 

When white flies were shown in 1946 
to be vectors of a vims of abutilon, 
a puzzle of long standing was partly 
solved* Variegated abutilon had been 


25 

used as an ornamental for many years 
in Europe and other parts of the world 
where the variegation never spread 
naturally to nonvariegated plants. 

Since there was no natural spread of 
the condition, the disease was some- 
times set somewhat apart from other 
virus diseases even though the variega- 
tion could be transmitted by grafting. 
It is now evident that in some countries 
a virus causing a similar variegation is 
disseminated in the field by a white fly. 

Mealybugs, tended and transported 
by ants, are the vectors of destmetive 
viruses that attack cocoa trees. Some 
of the vimses are closely related; in 
other cases relationships are uncertain. 
The vimses, which are not transmis- 
sible to plants by juice inoculations, do 
not remain long in the mealybugs un- 
less they fast before the acquisition 
feeding; then vims may be retained 
about 36 hours. In spite of the nonper- 
sistence of the virus, there is some 
specificity of transmission-certain 
mealybug species transmit certain of 
tlie vims strains not transmitted by 
others, and vice versa. Moreover, some 
strains of one of the mealybug species 
failed to transmit a vims that other 
strains of the same species transmitted. 

The only virus definitely known to 
be transmitted by thrips is the one that 
causes tomato spotted wilt. Forty-one 
diflereiit plant viruses have been tested 
for transmission by thrips with negative 
results. Although three species of thrips 
arc vectors of tomato spotted wilt, 
several other thrips species are not. 
Tomato spotted wilt vims can be ac- 
quired only by larvae, but it passes 
through the pupal stage, so that both 
adults and larvae can inoculate plants. 
After an incubation pet;iod of 5 days 
or more the insects remain infective 
for life. 

We have relatively few authenti- 
cated accounts of transmission by in- 
sects with biting mouth parts. In som<^ 
of these isolated cases, there is better 
transmission of the vims by biting 
insects than by those with sucking 



26 


YEAKBOOK OF AORICULTURB 1953 


mouth parts. The virus of turnip yel- 
low mosaic is an example of this kind 
of virus. It apparently is not trans- 
missible by insects with sucking mouth 
parts but is transmitted by a number 
of insects with biting mouth parts. The 
most important of these are flea beetles, 
the larvae of which may retain virus 
for as long as 4 days. It is believed 
that the insects regurgitate the virus. 

All viruses of this sort are readily 
transmitted by rubbing juice from 
diseased plants on the leaves of healthy 
ones, and some are readily isolated. 

Until recently the only viruses 
photographed under the electron mi- 
croscope were the more stable ones, 
which are i'eadily transmitted mechan- 
ically and occur in relatively high 
concentration in plants. Some are in- 
sect-transmitted. Among them are the 
tobacco mosaic and squash mosaic 
viruses. Less stable and less concen- 
trated viruses that have a more inti- 
mate relationship with their vectors 
have been identified recently under 
the electron microscope. Considerable 
interest attaches to the nature of those 
viruses that arc known to reproduce 
in both plants and animals. 

When one considers that the occur- 
rence of a virus disease of plants usu- 
ally involves three entities — the virus, 
the plant, and the vector — and may 
involve more than one of each, it 
should at once be apparent that the 
interactions between them and their 
environment may be exceedingly com- 
plex. In the laboratory, certain single 
factors may be demon.stratcd to be 
decisively important. Thus the virus 
of aster yellows is inactivated at 89® F, 
But when one tries to explain the 
vagaries of the spread of virus diseases 
in the field, relationships arc not al- 
ways readily discerned. For example: 
Northern regions and high altitudes 
with climates inimicablc to aphids 
generally favor the production of 
potatoes with a low virus content, but 
hot and dry climates arc also unfavor- 
able for aphids and under certain con- 
ditions can be used to produce seed 


potatoes with a low incidence of virus. 

The flight habits of aphids in the 
field in relation to the spread of po- 
tato virus diseases have been exten- 
sively studied at Rothamsted in Eng- 
land and the Maine Agricultural Ex- 
periment Station. It was determined 
in Maine that early and sustained 
flights containing a high proportion of 
Myzus persicae^ an aphid vector of leaf 
roll disease of potatoes, were asso- 
ciated with considerable spread of the 
disease. But flights late in August or 
in September usually resulted in little 
or no spread. 

Detailed work has been done on the 
ecology of the vector of the curly top 
virus. This leafhopper is an active 
flier, and large numbers can easily be 
borne long distances by the wind. It 
often multiplies in the spring on a 
variety of succulent weeds on unculti- 
vated or abandoned lands. If those 
plants dry up after the insects have 
reached the winged adult stage, the 
insects take flight. One such migratory 
flight was estimated at 60 miles. Fore- 
casts of leafliopper invasions based on 
studies of the breeding areas have 
been used to reduce losses. 

In general the insect vectors of a 
virus tend to be confined to one of 
the major phylogenetic divisions, such 
as the families of the Hemiptera or the 
order Thysanoptera. Some of the 
viruses multiply in their insect carriers 
and more may be expected to be 
placed in this category. The vector 
relationships of such viruses arc obvi- 
ously quite different from those of the 
viruses that do not multiply in the 
vector. Although more extensive 
research on certain viruses has greatly 
increased the number of known vec- 
tors for each, other research has at the 
same time indicated more highly spe- 
cific virus-vector relationships than 
were indicated in early work. 

L. M. Black is a projessor in the de^ 
partment of botany in the University of 
Illinois, Before joining the University 
ip ^952- he was curator of plant pathology 
in the Brooklyn Botanic Garden. 



THE FUNGI All LIVING OtGAWISMS 


The Fungi 
Are Living 
Organisms 

Russell B, Stevens 

Most fungi can be easily seen. Some 
are large and conspicuous, like the 
fleshy mushrooms of field and forest. 

Some are stout and woody, like the 
bracket fungi on rotting timber. The 
pathogenic ihingi, the ones that pro- 
duce disease, more closely resemble 
the molds that overrun our shoes and 
stored leather goods in damp weather, 
damage foods, rot fabrics, and, in 
happier circumstances, produce valu- 
able drugs and chemicals and give the 
desired flavor to our choicest cheeses. 

The most important detail to re- 
member about fungi is tliat they are 
living organisms. They require, there- 
fore, a source of energy. Like animals, 
but unlike green plants, fungi cannot 
convert the energy of the sun and thus 
synthesize the food they need. They 
must therefore get food from some ex- 
ternal source. In so doing, the slender, 
infinitely tiny, often colorless threads 
that make up the fungus itself live and 
grow at the expense of the environ- 
ment in which they exist. If the en- 
vironment is another plant, particu- 
larly if it is one of our cultivated agri- 
cultural plants, the resulting situation 
is regarded as a plant disease. Thus 
the apple scab fungus, familiar to any- 
one who has tried to raise apples in 
practically any part of the United 
States, forms olive-green patches on 
the leaves and fruits of its host and 
profits at its expense. The same is true 
of the grain rust fungi, the com smut 
fiingus, and a myriad of others. They 


27 

grow thus because by their very na- 
ture they can grow successfully no- 
where else. 

A pathogenic fungus is but one of 
countless individuals in the organic 
world. To maintain its position it must 
live and grow, reproduce, be scattered 
about, and, under certain circum- 
stances, survive periods during which 
the environment is relatively unfavor- 
able to it. When one Imows the 
essentials of how fungi accomplish this 
assignment, one knows the essentials 
of how fungi cause disease in plants. 

Structurally, fungi are mostly very 
tiny — but very complex. Most of them 
are composed of microscopic fila- 
ments, which may be colorless or vari- 
ously colored but which never contain 
chlorophyll, the green pigment re- 
sponsible for binding the sun’s energy 
in the formation of sugar. Considered 
individually, the filaments are called 
hyphae (singular; hypha). In the aggre- 
gate they are spoken of as a mycelium. 
The filaments are essentially like the 
cells of which other plants are com- 
posed, each being surrounded by a 
thin membranous wall and composed 
of a jelly like living protoplasm. 

Although the filamentous growth 
form is common to practically all plant 
pathogenic fungi, the actual appear- 
ance of the fungi is enormously varied. 

Some, like the rhizopus soft rot 
fungi of sweetpotatoes and strawberries 
or the scab fungi of wheat, have no 
particularly characteristic fonn and 
appear as a downy growth over the 
surface of the affected host. 

Others develop aggregates of fila- 
ments with a recognizable size and 
shape, of distinctive color, and often 
decidedly firm to the touch. Such 
aggregates are commonly associated 
with the actively reproducing phase of 
development, and the “fruiting body” 
so formed serves as one of the most 
useful means of distinguishing one 
plant pathogenic fungus from another. 

The physiology of fungi (like the 
physiology of all plants and animals) 
is by no means completely understood. 
We have enough information to sug- 



YIAttOOK Of AGKICULTURI 1953 


28 

gest that the physical and chemical 
processes that take place in the proto- 
plasm of the fungi are essentially the 
same as those of any other living 
organbm. Indeed, those processes, 
summed up in the term metabolism, 
arc strikingly similar throughout the 
organic world. Because the fungi lack 
chlorophyll, their nutrition is most like 
that to be found in the colorless, or 
rather tiongreen, parts of higher plants, 
except that many fungi secrete diges- 
tive enzymes as do the cells of the diges- 
tive tract of man and other animals. 

Because iungi depend for their food 
on outside sources, and because some 
establish themselves on other living 
plants and some on nonliving food 
sources, a great deal has been made of 
the terms parasite and saprophyte^ 
supposedly to describe these two alter- 
native conditions. Attempts to apply 
the terms soon demonstrate their inad- 
equacy, at least when used in unmod- 
ified form. It is much more useful to 
speak of obligate parasites, which can 
grow only on living host plants, and 
facultative parasites, which can grow on 
nonliving food supplies but which at 
times do establish themselves on a 
living host. True saprophytes, which 
live exclusively on nonliving substrata, 
can hardly be of importance in a 
discussion of plant diseases. 

Besides an ability to utilize food and 
grow, a fungus must reproduce. Be- 
cause it is the most intriguing phase 
of the life history and because it fur- 
nishes most of the information neces- 
sary for identification, the reproduc- 
tive aspects of fungi have received 
much attention. 

By reproduction we ought always to 
mean an increase in the total number 
of individuals of a given species, a 
process which usually represents suc- 
cessive generations and thus assures the 
perpetuation of the species in time as 
well as its maintenance in space. It is 
an error in logic, however, to conclude 
that fungi reproduce in order to sur- 
vive; one can say simply that they 
survive becait^te they reproduce. 

Characteristically, fungi reproduce 


by the formation of microscopic bodies 
collectively called spores. 

Such spores, depending on the type 
and the particular pathogen by which 
formed, assume an impressive array of 
different characteristics. They vary 
greatly in size, although always tiny in 
terms of more familiar objects, meas- 
uring several hundreds or even thous- 
ands to the inch. Some spores are one- 
celled. Some are two-celled. Some have 
many cells. Some are completely 
colorless. Some are only lightly tinted, 
others dark. Still others are co^ black. 
Some are extremely fragile and arc 
killed by exposure to dry air in less 
than a minute. Some can withstand 
boiling temperatures for brief periods. 
A few types are capable of independ- 
ent movement, but most of them drift 
passively on wind or water, although 
often they are forcibly ejected when 
first mature. Finally, they arc formed 
in myriad different ways by the parent 
organism. Detailed consideration of 
specific diseases is by all odds the best 
way to comprehend the complexity 
and variability of spore formation. 

No one, be he beginner or experi- 
enced professional, can truly appreci- 
ate the astonishing abundance in 
which spores are formed by fungi, 
even though they are produced on 
fruiting bodies so small as to be 
scarcely visible to the unaided eye. 
Total numbers from a single diseased 
plant must be estimated in millions, 
billions, even trillions, yet so little is 
the likelihood of survival that only the 
tiniest fraction of them are destined to 
give rise to a subsequent generation. 

Fungus spores arc often formed 
vcgetatively; that is, they develop at 
the ends of the filaments by constric- 
tion of the walls, or their appearance 
may follow as the direct result, more 
or less, of some form of sexual activity. 
Those of the former type may occur 
pretty much at random over the sur- 
face of the relatively undifferentiated 
mass of hyphae, or they may be more 
or less enclosed in a fruiting body, 
which in turn is made up of nonspore- 
bearing filaments. 



THE FUNGI AiS LIVING OtGANISMS 


The manner in which the spores are 
produced, and the appearance of the 
fruiting body, is the primary type of 
information upon which virtually all 
classification schemes are based. This 
is particularly true of the sexual spores. 
We thus have four groups of fungi: 

The Phycomycetes — the filaments 
have no cross walls, and sexual activity 
consists of the fusion of many-nucleated 
male and female cells. In this group 
alone the nonsexual spores are fre- 
quently capable of swimming about, 
and these fungi arc primarily found in 
very moist situations. 

The Ascomycetes — sexual spores, 
typically in groups of eight, arc to be 
found within a tiny sac, or ascus. 

The Basidiomycetes — sexual spores, 
two or four, arc borne upon the end of 
a tiny club, or basidium. The rust and 
smut fungi, which make up an impor- 
tant segment of this group, have a 
modified type of basidium clearly 
homologous to the true basidium. 

The Fungi imperfecti — only the 
nonsexual type of spore is known for 
this group, which otherwise greatly 
resembles the Ascomycetes. 

Within the group of the Ascomy- 
cetes, particularly, further subdivi- 
sions arc made on the basis of the type 
of fruiting body in association with 
which the sexual spores are produced. 

The nonsexual type of spore forma- 
tion generally is typical of the early 
stages of disease, often during the 
active growing season. 

Sexual reproduction, on the other 
hand, characteristically occurs in the 
later phases of disease development, 
prcliminarN' to a period of dormancy 
during the winter. Of the common 
diseases, the brown rot of stone fruits 
illustrates the point as well as any. 

Here, the fruit as it approaches 
maturity will be literally enshrouded 
with a soft, velvety mass of nonsexual 
spores, millions of them, which are 
responsible for the rapid spread of the 
pathogen during the summer. Only 
after the decayed fruit has shriveled, 
dropped, and lain on the ground for a 


29 

full winter do the fragile trumpets of 
the fruiting bodies appear, l:^aring 
their countless sacs of ascospores. 
These, in turn, carry the disease back 
to the budding and blossoming trees in 
springtime. 

Not only must an organism survive 
in point of time; it must also be 
distributed in space. With the excep- 
tion of fragile motile cells in certain 
Phycomycetes (and then only for very 
limited absolute distances) fungi are 
not self-propelled. They depend, there- 
fore, on chance distribution by exterior 
forces — wind, water movement, activ- 
ity of birds and animals, insect carriers, 
dissemination of host plants and plant 
parts, and so on. To a large extent the 
dissemination of fungi is through the 
medium of their spores, tiny objects 
often ideally suited for the purpose. 
Fragments of hyphae, hard masses of 
mycelium known as sclerotia (singu- 
ular: sclerotium), and possibly other 
entities account for the remainder. 

Spore movement in the bulk of 
fungus species is passive, although 
flagellated forms occur in a few species 
and in many others the initial dis- 
charge is forcible. The distance to 
which fungus spores may be carried 
by air currents, for example, is very 
great. Because of this, and because of 
the astonishing productivity of some 
pathogens, we are witness to occa- 
sional dramatic epidemics of disease. 
No better illustrations of this exist in 
the United States than the northward 
sweep of the cereal stem rusts in the 
Great Plains area, and the march of 
potato late blight, tobacco blue mold, 
and cucurbit downy mildew up the 
eastern seaboard. 

Assuming the successjEul growth, re- 
production, and the dissemination of 
fungi, and their establishment upon 
a host plant, there must yet be times 
when their continued existence depends 
upon their surviving a period gC 
relatively unfavorable conditions. This 
can mean either extremes of tempera- 
ture, moisture deficiency, oxygen in- 
adequacy, or simply a period during 



30 YEARBOOK OF A 

which there are no available host 
plants. Survival may depend upon the 
presence of thick-walled spores, of 
sclerotia, or upon the ability to live 
for some time as a saprophyte upon 
the organic matter in the soil. 

Much emphasis is placed, in teach- 
ing plant pathology, for example, on 
the life history, or life cycle, of each 
fungus. On that basis, one may think 
of diseases as developing during that 
part of the life of a fungus when it is 
actively growing in association with a 
host plant. 

Pathogenic disease, after all, is not 
a thing but a condition — a state of 
unstable equilibrium that results from 
the interaction of at least two living 
organisms. 

Numerous technical terms have 
come into wide usage. A few at least 
are instructive: 

Inoculation — tha t c ircumstance where- 
by the pathogen and host come inti- 
mately into contact, as by the falling 
of an airborne spore onto a leaf sur- 
face. Inoculation might also be ac- 
complished through the intervention 
of man or animal. 

Invasion — entry, passive or active, of 
the pathogen into the host. 

Establishment — development by the 
fungus of a successful association with 
the host tissues, enabling further de- 
velopment of the pathogen. 

Incubation — development of patho- 
gen, in association with the host, prior 
to the appearance of conspicuous, 
recognizable symptoms of disease. 

Characteristics of host (age, abun- 
dance, presence of wounds, degree of 
resistance) as well as of the pathogen 
enter into the above-mentioned events, 
as do considerations of the environ- 
ment. Not infrequently a third organ- 
ism — man, insect, bird, nematode, or 
other animal — may have a decisive 
role. Whatever the factors involved, 
and at whatever stage, the final de- 
velopment of well-defined disease is 
the result of a vast complex of ele- 
ments. 

Sporulation, important as it is from 


RICULTURE 1953 

other viewpoints, is of little significance 
with regard to the particular host 
plant involved. Damage is done dur- 
ing the essentially vegetative phase of 
pathogen development and is largely 
realized by the time spore formation 
commences. 

The very core of the whole problem 
of fungi as causal agents of plant dis- 
ease lies in the phrase “host-parasite 
relation.” It is disappointing, but not 
surprising, to realize that of all phases 
this is pre.sently the least well known. 
We do not even know, in the vast 
majority of cases, in just what way a 
given pathogen is harmful to its host, 
whether it is a matter of food compe- 
tition, toxin secretion, enzyme dis- 
turbance, or the like. Some specific 
considerations are nonetheless useful. 

Parasitism may be cither obligate, 
in the sense that the pathogen will not 
grow actively save on living tissue; 
or facultative, indicating that it may 
grow well at certain stages of its life 
cycle on nonliving organic matter. 
The obligate parasites usually have a 
very specific association with their 
hosts, the cells of the latter being 
invaded l;y microscopic outgrowths 
of the hyphae called haustoria. These 
are thought, but not fully proved, to be 
absorptive in function. Facultative 
parasites, on the other hand, often 
injure by the secretion of extracellular 
enzymes. 

Certain fungi are strictly local in 
their effect, producing lesions on leaf, 
stem, or root system, though their 
localism may at times be an expression 
of host resistance. Other fungi are 
selective with respect to particular 
tissues, as exemplified by the va.scu]ar 
wilt organisms, which are confined 
to the water-conducting tissues, or the 
chestnut blight fungus, injuring the 
cambium layer. Still others are indis- 
criminate, establishing themselves at 
various points, and at times destroying 
the entire plant. 

There is the anticipated correlation 
between the mode of dissemination of 
the fungus and its relation to the host. 



IDENTIFYING A PATHOGENIC FUNGUS 


31 


In general, leaf, stem, and fruit dis- 
eases are caused by airborne or insect- 
carried fungi, root diseases by soil- 
inhabiting species. Some vascular wilts 
are caused by soil fungi, some by fungi 
possessing insect vectors. 

While there is probably no really 
simple host-pathogen relation, the 
complexity achieved in certain in- 
stances is truly impressive. Possibly the 
best understood instances of a complex 
interrelationship are to be found in the 
so-called heteroccious rusts. Here one 
is confronted with a pathogen that is an 
obligate parasite, having as many as 
five distinct spore types, and compelled 
to alternate from season to season be- 
tween two botanically very difierent 
host species. How such a situation 
evolved over the past ages remains a 
complete mystery. 

It goes almost without saying that 
critically accurate knowledge of the de- 
tails of pathogen life cycles is essential 
to the development and application of 
effective control measures. 

The attention given the fungi as 
causes of plant disease seems in large 
measure due to two further charactci - 
istics. In the first place, it is the fungus 
diseases of plants (by contrast w'ith 
those of bacterial and virus origin) that 
are most easily controlled by chemical 
applications in the form of sprays and 
dusts. Added to this is the fact tliat 
fungi arc responsible for a much larger 
number of the rapidly spreading, hence 
epidemic, diseases than are viruses or 
bacteria. 

Whatever the reason, it is the spo- 
radic dhseases of this nature that bring 
about the greatest hardships on the in- 
dividual farmer. Small wonder then 
that our most publicized maladies are 
wheat rust, apple scab, potato blight, 
and the like. 

Russell B. Stevens is associate pro- 
fessor of botany in the University of Ten- 
nessee, He obtained his doctorate at the 
University of Wisconsin in ig^o and served 
in the Army Medical Department during the 
Second World War, He is author with his 
late father of a textbook^ Disease in Plants. 


Identifying a 

Pathogenic 

Fungus 

William W, Diehl 

From the beginning to the end of its 
life the health of every seed plant, wild 
or cultivated, is affected by fungi. 

Even though a seed within a fruit or 
capsule may be sterile, it comes into 
contact with fungal spores and hyphac 
as soon as it is exposed to the air or is 
in contact with the ground. Spores 
arc micro.scopic, seedlike, reproductive 
bodies, and hyphae are the micro- 
scopic vegative growths of fungi. 

The air is literally charged with 
spores, and the soils of the whole earth 
are full of living spores and hyphae of 
different kinds of fungi. Most of the 
fungi are innocuous. Many are bene- 
ficial. But some thousands of recog- 
nizably different kinds of fungi are now 
known to be pathogens, or agents of 
disease, in plants. 

Practical measures for the prevention 
and control of plant diseases depends 
in large part upon scientific knowledge 
of each pathogen and its role in nature. 
Since there are more than 100,000 
recorded names of supposedly different 
kinds or species of fungi, the specific 
identification of a single specimen or 
culture of a fungus involves the exclu- 
sion of some 99,999 naSnes. That is a 
technical problem akin in complexity 
and difficulty to the isolation and iden- 
tification of any one out of 100,000 
chemical compounds. 

But the problem is not insuperable. 
There is a general procedure that leads 
the way out of the apparent chaos of 
more than 100,000 names. 



32 YEA.BOOK OF A 

First of all the specimen must be 
subjected to critical examination in 
order to determine any and all features 
that characterize it. Spores and fnic- 
dheations, or spwre-bearing structures, 
are the most significant features for 
diagnostic purposes. 

To sec the features to best advantage 
under the different powers of the com- 
pound microscope requires special 
preparation for each kind of material. 
The form as well as the texture of a 
fungal fructification, whether moldlike 
and fluffy or a solid structure, will 
determine the best method of treat- 
ment. 

Most fructifications of microfungi are 
best viewed at first in place by reflected 
light with a hand lens or, better yet, a 
stereoscopic microscope, followed by 
examinadon under the different pow- 
ers of the compound microscope of a 
very minute fragment mounted in 
water or in a staining medium. 

Molds arc more or less easily 
mounted in water or special mounting 
media, although they frequendy re- 
quire preliminary treatment with a 
^ing fluid to prevent a loss of spore 
heads, chains, or other delicately 
attached structurc.s. Nevertheless, if 
immature stages are placed directly in 
the mountant, those structures that are 
too readily detached when mature 
often tend to remain in place so that 
their genesis is more readily observed. 

If a fructification is large and opaque 
its anatomy can be discerned only 
in sections. Microtome sections made 
from materials imbedded in paraffin 
or nitrocellulose are the acme in ele- 
gance and are essential if good photo- 
micrographic records are desired. 
Under ordinary circumstances, how- 
ever, their preparation is too time- 
consuming to be justified, since for 
most practical purposes satisfactory 
sections are quicldy made free-hand or 
by means of the freezing microtome. 
With moderate practice, excellent free- 
hand sections can be made using elder 
pith, carrot, or other convenient plant 
material as a clamp to hold a fragment 
firmly while slicing a number of sec- 


IRICULTURE 1953 

tions among which only the best need 
be selected for study. If the material is 
too scanty to permit wastage or if the 
operator has not mastered the more 
rapid technique of free-hand section- 
ing, recourse may be had to the freez- 
ing microtome. Although a second- 
rate instrument as microtomes go, it 
has its advantages. Fungal structures 
that arc too hard for easy sectioning or, 
after sectioning, arc too impenetrable 
to transmitted light may usually be 
softened or cleared by soaking for a 
suitable period in some softener or 
clearing agent, such as a solution of 
potassium hydroxide or chloral hy- 
drate. Clearing agents effectively re- 
move fats and oils. Often after their 
use, structural details not otherwise 
evident are rendered more distinguish- 
able, especially if they are stained. Cer- 
tain mounting media, which clear and 
stadn at one operation, are distinctly 
advantageous although the unstained 
aqueous mounts are usually satis- 
factory, especially so for water molds if 
the microscope illuminant is properly 
adjusted. A pha.se microscope is of 
decided advantage for living materials. 

If the living specimen or culture 
possesses well-marked, matured spore- 
bearing structures, it is usually ade- 
quate for study. But if it bears no 
fructifications or only immature ones, 
they may often be produced or forced 
into recognizable maturity by such 
expedients as immersing them in 
water or keeping them in a moist 
atmosphere for a convenient period. 
Moist chambers arc easily improvised 
by placing wet blotting paper under 
a bell jar or in a closed mason jar. 
It is sometimes preferable to keep 
specimens moist by having them 
wrapped in a wet towel. If there is 
any likelihood that the fungus requires 
an especially low or high temperature 
for maturation, that condition should 
be met. 

Diagnostic features of many fungi 
are best developed through pure cul- 
ture on selected artificial media in 
petri dishes. Standard media are par- 
ticular combinations of nutrients and 



IDENTIFYING A PATHOGENIC FUNGUS 


agar gel, but there is a wide choice of 
formulas. The growth reactions of 
some species arc characteristic on cer- 
tain media and not on others. In pure 
culture on artificial media a species 
may, hirthermore, present a different 
app>earance from that in nature. 
Hence it may be needful to grow it 
upon the natural substrate to obtain 
the development of normal fructifi- 
cations. 

Having observed and interpreted 
the more significant morphological 
features, one records them, at least 
tentatively, by means of sketches and 
notes. One pays special attention to 
measurements. 

In general, spores and spore-bearing 
structures are preferably measured in 
water mounts, because published de- 
scriptions of these features have usually 
included dimensions determined from 
material mounted in water. The re- 
corded characters are then utilized in 
tracing through analytical keys of the 
fungi to the several classes, orders, 
families, and genera, and finally to a 
species. 

There are several standard keys in 
general use that lead to families and 
genera. G. W. Martin’s key to families 
in the very useful Dictionary of the 
Fungi (third edition, 1950), by G. C. 
Ainsworth and G. R. Bisby, is a sim- 
plified and modern presentation, but 
for keys to genera one is forced to 
seek elsewhere. The keys to be found 
in E. A. Bessey’s Morphology and Tax- 
onomy of the Fungi (1950) are valuable 
for teaching purposes, but lead to 
representative genera only. The key to 
The Genera oj Fungi (1931), by F. E. 
Clements and C. L. Shear, and those 
to be found in Englcr and PrantFs 
Pfaturlkhen Pflanzenfamihen (1897- 
1900), although today somewhat out- 
moded, arc still essential references. 

When a decision is reached as to the 
genus to which the fungus under con- 
sideration belongs, the problem re- 
mains of finding suitable literature 
bearing specific descriptions. The 
Guide to the Literature oJ the Fungi, the 
last chapter in Bessey’s book, lists the 


33 

more useful monographs and com- 
pendia as references. Yet one cannot 
depend upon compendia and mono- 
graphs alone. They are out of date as 
soon as printed. It is therefore neces- 
sary to take account of the numerous 
increments constantly being published 
— hence the need for access to well- 
cataloged library facilities. 

Host indexes as short cuts are legiti- 
mate aids in quickly finding specific 
names that might apply. A pathogen 
may, of course, have thus far escaped 
record as upon the particular host, 
but it is likely to be recorded, if at all, 
on some related host. A. B. l^ymour’s 
Host Index (1929), based upon a com- 
plete but unpublished catalog of rec- 
ords up to 1924 and partly through 
1926, is supplemented by the later 
detailed cumulative Index oJ Plant 
Diseases in the United States (1950) by 
F. Weiss and (1952, 1953) by F. Weiss 
and M. J. O’Brien. Various foreign 
lists of fungi and plant diseases, no- 
tably the anonymous List of Common 
British Plant Diseases (University Press, 
Cambridge, 1944) and the Enumeratio 
Fungorum (1919-1924), by C. A. J. A. 
Oudemans, are useful because most 
fungi tend to be cosmopolitan. 

Actually, host indexes, like regional 
lists, are merely suggestions in deter- 
mining identities, and one must ulti- 
mately depend on monographs, sup- 
plemented by the comparisons with 
herbarium specimens, including cited 
fungi exsiccati. Fungi cxsiccati are stand- 
ard replicate herbarium specimens of 
definite reference value, but compari- 
sons with authentic specimens and 
with types constitutes a court of last 
resort. 

Considerable information on taxo- 
nomic techniques with fungi is to be 
found in G. R. Bisby’s Introduction to 
the Taxonomy and Nomenclature of the 
Fungi (1945) and in M. Langeron’s 
Precis de Mycologie (1945)- Whether a 
fungus in culture is an exact replicate 
of a species with ample record of 
pathogenicity can of course be deter- 
mined only by means of culture studies 
with inoculation experiments in order 



YBARBOOK OF AOtlCULTURE 1953 


34 

to reveal comparable growth reactions 
and host symptoms. 

When the identity of the fungus 
seems assured, there is still the ques« 
tion whether its name is acceptable. 
Even if a specific name (epithet) has 
been found entirely applicable to the 
specimen at hand — that is, its fea- 
tures agree in all details with those 
noted in the description and it very 
closely resembles the type and other 
specimens regarded as authentic — 
there is always the likelihood that there 
may be other names (synonyms) that 
might apply equally well. If one or 
more names are found to be synonyms, 
a decision must be made as to which 
is the correct one to use, according to 
the current International Code of Botanical 
Nomenclature ( 1 952) . Each of the several 
synonyms may be valid — that is, it has 
been properly published — but con- 
formity with the Code will determine 
which combination of generic and spe- 
cific names is legitimate and, therefore, 
the proper choice. The present Code 
epitomizes the evolution in the nomen- 
clature of fungi that began with the 
pioneer w^ork of the eighteenth century. 

Some persons, even specialists them- 
selves, at times assume that the most 
expeditious w'ay to get specimens or 
cultures identified is to refer them to 
individuals in other institutions. If the 
recipient is both competent and oblig- 
ing, he is soon so overburdened w'ith 
requests, many of them trivial but time 
consuming, that his own eficctiveness 
in service and research is vitiated. Ac- 
tually, the number of experienced my- 
cologists equipped with laboratory, 
catalogs, and library facilities adequate 
for this kind of service in the United 
States, or in any other country for that 
matter, is limited to a few persons in a 
few in.slitutions. 

The taxonomist’s concern, as well as 
his experience, is generally limited to 
particular genera or families. He nat- 
urally welcomes specimens and cul- 
tures that apply to his specialty; for 
him they arc relatively easy to deter- 
mine or else they challenge his mettle. 
Of course, no taxonomist can avoid a 


certain amount of drudgery; yet he 
should not be expected to determine 
the many common pathogens that 
ought to be^ more familiar to the send- 
ers. Some fairly common pathogens 
are often less familiar to tlie mycologist 
and can pose for him as much of a 
problem as any other unknowm fungus. 
The sender is morally obligated to 
explain the significance and impor- 
tance of his request, to supply the spec- 
imen or culture in good condition and 
in ample amount, as well as to accom- 
pany it with all pertinent data: Sub- 
strate, locality, date, etc. Since speci- 
mens and even cultures too often in- 
clude more than one organism always 
possible as later contaminants, the 
sender sliould send microscopic prepa- 
rations and sketches, sometimes even 
photographs, to avoid any possible 
confusion. Obviously, any materials 
entrusted to the mails should be so 
prepared that on receipt they w'ill be 
in good order and not an unrecogniz- 
able mass mixed with broken glass. 

As Bisby has remarked, “it is a matter 
of professional etiquette not to send 
parts of the same colicction [or dupli- 
cate cultures] to be named by different 
experts”— to which may be added, 
“unless the different exficrts are so 
notified.” It is almost universally con- 
sidered unethical for one to publish 
without acknowledgment a detenni- 
natiun provided by another. It is fur- 
thermore a convention that, lacking 
special agreement to the contrary, any 
specimens sent to another for deter- 
mination become the property of the 
recipient for deposit in the herbarium 
where he is employed and that he he^ 
the right to publish at his own discre- 
tion the result of his researches upon 
such materials. 

William W. Diehl is a mycologist in 
the division of mycology and disease surv^ 
at the Plant Iridustry Station, Beltsville, 
Md. He has served the Department of Agri^ 
culture in Washington and at Beltsville 
since igiy. He is a graduate of Miami 
University and holds advanced degrees from 
Iowa State College and Harvard University. 



PROBLEMS OF VARIABILITY IN FUNGI 


Problems of 
Variability 
in Fungi 

E. C. Stakman, J, J. Christensen 

Many plant disease fungi are known 
to be complex and variable, but the 
degree of complexity is imperfectly 
known for most of them and is not 
well enough known for any of them. 

This complexity is not surprising 
because many of the crop plants in 
which the fungi live are equally 
complex. There are more than 14,000 
varieties of wheat; there are un- 
numbered varieties of apples, beans, 
corn, roses, iris, and other cultivated 
and wild plants. Moreover, man and 
Nature arc continually making new 
varieties by mutation and by hybridi- 
zation and selection. 

Plant disf'ase fungi also are plants. 
They are parasitic on higher plants. 
Most are microscopic in size and 
relatively simple in structure, but they 
are plants nevertheless. They germi- 
nate, grow, and fructify; they mul- 
tiply by the countless billions; they 
mutate and hybridize; consequently 
Nature has made countless kinds in 
the past and is continually making 
more kinds through mutation and 
hybridization. As plant breeders pro- 
duce better kinds of useful plants, 
therefore, Nature may produce more 
destructive fungi. It is the job of plant 
pathologists to learn what the para- 
sitic fungi are now doing and what 
they may do in the future to jeopardize 
or nullify man’s success in plant im- 
provement. To understand present 
plant disease situations and to predict 
possible future developments, it is 


35 

essential to learn as much as possible 
about the variability in fungi patho- 
genic to plants. Other kinds of causal 
agents of plant diseases, such as viruses 
and bacteria, must be studied sim- 
ilarly, but the present discussion is 
restricted to fungi. 

The principles of growth and re- 
production in plant pathogenic fungi 
arc similar to those for higher plants, 
except that fungi have no chlorophyll, 
which enables green plants to make 
their food from simple materials in 
the soil and air. The fungi therefore 
must live on living plants or animals 
or their products. The fungus parasites 
of plants live in or on living plants and 
get their nourishment from them by 
means of minute tubes, or hyphae, 
that grow, branch, and form a net- 
work known as the mycelium, which 
finally produces many tiny spores. 

Spores serve the same propagative 
and reproductive function as the seeds 
of higher plants, but spores arc much 
smaller and simpler in structure than 
seeds. Spores usually have one or at 
most a few cells and do not contain 
an embryo as do seeds. Nevertheless 
they germinate and produce new 
fungus plants with the parental char- 
acters. The conditions required for 
germination arc similar to those for 
seeds, moisture and suitable tempera- 
ture being the most important. The 
time needed for germination varies 
with the kind of spore and the tem- 
perature; it varies from about an hour 
to several days. Many fungi produce 
millions or even billions of spores in a 
few days to a few weeks and can there- 
fore produce countless numbers in a 
few short generations. That is one 
reason why they are so dangerous. 

Some spores are produced asexually 
and some only as a result of sexual 
fusions. The same fungus may produce 
several kinds of spores asexually, but 
usually produces only one kind sexu- 
ally. Successive crops of asexual sporCiS 
are generally produced abundantly 
and quickly when conditions favor 
growth of the fungus, and the sexual 
spores are usually formed when the 



YEARBOOK OF AOftICUlTURE 1953 


36 

vegetative period is terminating. Each 
kind of spore has a special name; thus 
some of the rusts produce five kinds of 
spores or sp)orelike bodies — teliospores, 
sporidia, pycniospores, acciospores, and 
urcdosporcs, each with special form 
and function. 

The principles and procedures in 
classifying and naming fungi are simi- 
lar to those for higher plants. Species 
of higher plants arc determined by 
morphologic characters of the vegeta- 
tive and fruiting structures, including 
seeds. Similarly species of fungi are 
determined by the characters of the 
mycelium, of the spore-producing or- 
gans, and of the spores themselves. The 
unit of measurement for fungus spores 
is the micron, which is one one-thou- 
sandth of a millimeter; 25 millimeters 
make an inch. The range in size is 
from about 4 microns to more than 
100 microns and, together with shape, 
color, and certain other characters, is 
characteristic for each species. 

Visible characters are used for group- 
ing higher plants and plant pathogenic 
fungi into classes, orders, families, gen- 
era, species, and varieties. Thus it Ls 
easy to distinguish larger groups of 
agricultural plants, such a.s grasses and 
legumes. It is equally easy to distin- 
guish larger groups of fungi, such as 
smuts and rusts. But there are many 
kinds of wild and cultivated grasses, 
such as timothy, bromegrasses, oats, 
barley, rye, rice, corn, and wheal; and 
there arc many kinds of legumes, such 
as beans, peas, soybeans, alfalfa, and 
clover. Likewise, there are many kinds 
of smuts, such as the loose smut of 
wheat, the stinking smuts of wheat, 
common corn smut, and rye smut, and 
there are more than 3,000 kinds of 
plant rusts. It is relatively ea.sy to rec- 
ognize these major groups of higher 
plants and of fungi, but it becomes in- 
creasingly harder to classify the smaller 
groups, such as species and varieties. 

The agronomist must know not only 
wheat, a genus scientifically de.sig- 
nated by its Latin name Triticum^ but 
he must know the species of wheats, 
such as the common bread wheat, 7W- 


ticum vulgare; the durum wheats, T. 
durum; and several other species or sub- 
species. He must know also thousands 
of varieties of bread wheats and hun- 
dreds of varieties of durum. And he 
must learn alx)ut the new lines and 
potential varieties that continually are 
being produced. Likewise the plant 
pathologist must know the genera, 
species, varieties, and “lines” of the 
fungi that parasitize different kinds of 
wheat, and he must learn about the 
new ones that continually appear. 

Moreover, the agronomist must 
know not only what crop plants look 
like, but also how they behave and 
what they are good for. He must know 
whether a variety of wheat is a winter 
wheat or a spring wheat, whether it is 
winter hardy, where it grows well, 
whether it makes good flour or poor 
flour, and whether it is susceptible or 
resistant to disease; and the plant pa- 
thologist must know not only what spe- 
cies of plant disease fungi look like l.)ut 
must also know their pathogenicity for 
the many different kinds and varieties 
of plants. 

Most sp^ecies of plant pathogenic 
fungi arc equally as complex in com- 
position as sfjecies of crop plants, and 
it is therefore necessary to find out 
what is within the species. As an exam- 
ple, the species Puccinia graminis^ the 
stern rust of small grains and gra.sses, 
is recognized easily by its general ap- 
pearance and by certain cliaracters of 
its spores. But there are several varie- 
ties w ithin the species that are alike in 
some characters and difl'erent in others, 
including the kinds of crop) p)lants that 
they can attack. Thus the variety tritici 
of Puccinia graminis para.sitizcs wheat, 
barley, and many w'ild gras.ses; the 
variety aumae parasitizes oats and cer- 
tain w'ild grasses but not wheat and 
barley; w'ithin the tritici variety there 
are races that differ in their ability to 
attack certain varieties of wheat; with- 
in the variety avenae there are races that 
differ in ability to attack varieties of 
oats. Kanred, as an example, is im- 
mune to some races of the variety tritici 
and susceptible to others. These rust 



PROBLEMS OF VARIABILITY IN FUNGI 


races in turn can be subdivided into 
the ultimate subdivisions, the biotypes. 

A biotype is a population of indi- 
viduals that are identical genetically. 
The descendants of a single non- 
scxually produced stem rust spore 
constitute a biotype, unless mutation 
occurs to cause genetic diversity. The 
test of genetic purity of a population of 
a pathogenic fungus is the consistency 
of its behavior. Most species of plant 
pathc^enic fungi comprise many bio- 
types, and attempts are made to group 
the most closely related ones into races 
and then to group the most closely 
related races into varieties, which in 
turn are grouped into species. Actually 
the procedure usually is in the reverse 
direction: The larger groups arc recog- 
nized first, and the successively smaller 
groups are discovered by successively 
more refined methods. This can be 
illustrated by results of investigations 
of Puccinia gramims, the fungus that 
causes stem rust of small grains and 
gras.ses. (The discussion of stern rust is 
based on results of cooperative in- 
vestigations between the United States 
Department of Agriculture and the 
University of Minnesota, which were 
begun in 1914 and continued by many 
investigators under the general super- 
vision of the senior author, who regrets 
that the contributions of the many 
administrators and investigators can- 
not be specifically mentioned without 
interrupting the continuity of the 
discussion.) 

Puccinia graminis is a good species 
within which there are clearly recog- 
nizable varieties, races, and biotypes. 
There arc at least six rather distinct 
varieties that difl'er in size of spores 
and in the kinds of plants that they 
can attack: (i) tritici (wheat), whose 
host plants arc wheat, barley, and 
many wild grasses; (2) secalis (rye), rye, 
barley, and many wild grasses like 
those attacked by tritiev, (3) avenae 
(oats), oats and wild grasses different 
from those attacked by tritici\ (4) 
phleipratensis (timothy), timothy and 
certain wild grasses; (5) agrostidis 
(redtop), redtop and other species of 


37 

Agrostis\ (6) poae (bluegrass), Kentucky 
bluegrass (Poa pratensis) and related 
species. 

There probably are more varieties 
than the six listed, and new ones may 
appear in future as a result of mutation 
or hybridization. Races are known 
within the varieties tritici^ avenae^ and 
secalis. Although there sometimes arc 
at least slight morphologic differences 
between races within a variety, the 
most important and most easily recog- 
nizable differences are in the degree of 
infection (pathogenicity) on certain 
varieties within the genera Triticum 
(wheat), Avena (oats), and Secale (rye), 
respectively. The crop varieties, desig- 
nated as differential varieties, now 
used to distinguish races within the 
tritici and avenae varieties, were selected 
from hundreds that were tested, and, 
at the time of selection, appeared to be 
representative and adequate for dis- 
tinguishing races. 

'Fhc differential varieties of wheat 
arc: 

Triticum compactum (club wheats): 
Little Club, C. I. 4066 (Cereal In- 
vestigations accession number. De- 
partment of Agriculture). (Certain 
lines of Jenkin, C. I. 5177, notably 
Hood, C. I. 11456, may be substituted 
for Little Club.) 

Triticum vulgare (bread wheats): 
Marquis, C. I. 3641; Reliance, C. I. 
7370 (certain lines of Kanred, C. I. 
5146, can generally be substituted 
for Reliance); Kota, G. I. 5878. 

Triticum monococcum (Einkorn): Ein- 
korn, C. I. 2433. 

Triticum durum (durum or macaroni 
wheats): Arnautka, C. I. 1493; Min- 
dum, C. I. 5296; Spelmar, C. 1 . 6236; 
Kubanka, C. 1 . 2094; Acme, C. 1 . 
5284. 

Triticum dicoccum (cmiher); Vernal, 
C. 1 . 3686; Khapli, C. I. 4013. 

When inoculated with different 
races, these varieties may he sus- 
ceptible (S), resistant (R), or hetero- 
geneous (mesothetic) (M). Each of 
these classes is subdivided into infec- 
tion types, based on size of pustules 
(small iDlisterlike eruptions containing 



YEARBOOK Of AORICULTURE 1953 


38 

from a few to 250,000 rust spores), 
and the condition of the wheat tissue 
in which pustules are formed. The 
rust stage which is used in identifica- 
tion is the repeating or summer-spore 
stage, called also the urcdial stage; 
and the pustules of this stage are 
called also urcdia (singular, uredium). 

(0) Immune — No rust pustules de- 
veloped; small flecks of dead host tis- 
sue sometimes present and designated 
by a semicolon, thus, o;. 

(1) Very Resistant — Rust pustules 
extremely- small and surrounded by 
dead areas. 

(2) Moderately Resistant — Pustule 
small to medium; usually in green is- 
lands of host tissue surrounded by a 
band of yellowish (chlorotic) or dead 
tissue. 

(3) Moderately Susceptible — Pustules 
medium in size; usually separate, no 
dead areas, but yellowish (chlorotic) 
areas may be present, especially under 
unfavorable conditions. 

(4) Very Susceptible — Pustules large 
and often united; no dead tissue, but 
there may be leaf yellowing under un- 
favorable growing conditions. 

(x) Heterogeneous — Size of pustules 
variable, sometimes including all of 
the above types and intergradations 
between them on the same leaf. 

That gi'ouping indicates the types 
of infection produced by rust races 
on the differential varieties of wheat. 
As indicators of resistance or suscepti- 
bility of the wheat varieties, the infec- 
tion types are grouped as follows: 
Types o, i, and 2 are resistant (R); 
types 3 and 4 are susceptible types 

(S) ; and type x is mesothctic (variable) 
(M). 

It will be seen, therefore, that vari- 
eties are considered resistant when the 
rust is unable to grow extensively and 
produce pustules on them or when the 
pustules are extremely small, as indi- 
cated by types i and 2. The small 
pustules arc likely to be surrounded 
by discolored or dead host tissue, but 
the area*- killed arc almost always 
very small, so that the rust actually 


docs very little damajge to the variety. 
The production of infection types 3 
and 4 on varieties indicates that they 
are susceptible, because the rust can 
grow well and produce large pustules 
with large numbers of spores, without 
killing the host tissues of the wheat 
plant quickly and thus limiting its 
own growth, as the rust cannot grow 
in dead tissue. The infection type x 
is variable and therefore is an indica- 
tion that the resistance or suscepti- 
bility of the variety is also variable. 

To identify rust races, seedling plants 
of the 12 differential varieties arc in- 
oculated in the greenhouse by applying 
rust spores to them, after which the 
inoculated plants are kept in a moist 
chamber for 24 hours to provide mois- 
ture for the spores to germinate and 
cause infection. It takes i to 2 weeks 
for the rust to produce a new crop of 
spores. For the identification of races, 
only three reaction classes are used, 
namely: resistant (R), susceptible (S), 
and mesothctic (M). The races are 
designated by numbers; the differences 
between races 56 and 59 arc shown in 
the table: 


Little Club 

Race 

S 

Race 59 
S 

Marquis 

Rf liance 

S 

R 

S 

R 

Kota 

S 

R 

Amautka 

R 

R 

Minduin 

R 

R 

Spclinar 

R 

R 

Kubanka 

S 

M 

Acme 

S 

S 

Einkorn 

R 

s 

Vernal 

R 

" R 

Khapli 

R 

R 


The table shows that there arc de- 
cided differences between races 56 and 
59. Both races attack Little Club 
normally. Marquis, Reliance, and 
Kota are susceptible to race 56 and 
resistant to race 59. Amautka, Min- 
dum, and Spelmar are resistant to both 
races. They differ again on Kubanka 
and are the same on Acme. They differ 
on Einkom and produce the same 
effect on Vernal and Khapli. They 
produce the same effect on seven vari- 
eties but differ on five of them. They 



PROILiMS OF VARIABILITY IN FUNOI 


are therefore distinctly different races, 
as determined by their ability to attack 
certain varieties of wheat. 

Race 59 comprises several known 
biotypes. The collections of rust from 
the United States and Mexico that 
were identified by E. C. Stakman and 
colleagues as race 59 between 1929 
and 1944 produced the infection types 
given in the foregoing. Collections of 
rust taken from the field may contain 
several races, which are separated 
from each other by appropriate inoc- 
ulations on differential varieties and 
the different races can then be isolated 
and cultured separately. Each purified 
culture is called an isolate. During a 
season when many collections of rust 
are identified, many isolates of the 
same race are made, and a number of 
them usually arc kept for comparative 
tests at the same time and under the 
same conditions. 

In 1944 an isolate from a collection 
that was obtained directly from bar- 
berry bushes in Massachusetts produced 
infection type 2 on Reliance wheat in- 
stead of the type o, which was char- 
acteristic of the race; soon thereafter 
a similar isolate was obtained from a 
collection of rusted barberry leaves 
from Wasliington State. When com- 
parative tests were made between 
isolates that produced type o and 
those that produced type 2, the dif- 
ference proved to be consistent, in- 
dicating that the isolates were geneti- 
cally different. As the isolates could not 
be subdivided further, it was concluded 
that each was pure genetically, and 
they were considered as two biotypes 
of the same race, 59, because they 
were alike on 1 1 varieties and the 
difference on the one variety. Reliance, 
was slight; Reliance was immune to 
one and highly resistant to the other. 
The new biotype was therefore desig- 
nated as race 59A. Later in 1944 two 
other isolates, also from barberry, 
differed slightly but consistently from 
the other two on Kota wheat, and 
they were designated as 59B, and 59C, 
respectively. The infection types pro- 


39 

duced by the four biotypes on Re- 
liance and Kota are: 

Reliance Kota 

59 o o; 

59^* • • a o; 

59® 2 a 

59C o 2 

(The semicolon after o indicates that 
there were small flecks of dead wheat 
tissue, but no pustules.) 

These minor differences in infection 
types on Reliance and Kota seemed 
scientifically interesting rather than 
practically important until it was 
found that 59A is much more virulent 
than 59 on certain previously resistant 
varieties of barley. Clearly, then, the 
differences between at least two bio- 
types of race 59 on the differential 
varieties of wheat are not the only 
differences between them, and, if they 
could be tested on all known varieties 
of wheat and barley, additional dif- 
ferences might be found which would 
justify considering them as separate 
races. As new' varieties of W'heat arc 
produced, rust isolates that now appear 
identical may prove to be difi'erent. 
Race 15 is a good example. 

Race 1 5 of wheat stem rust was dis- 
covered in 1918. The race was easy to 
identify; all of the 12 differential 
varieties of wiieat except Khapli are 
susceptible to it. The identification of 
njst collections from different places 
in the United States disclosed that 
some isolates of race 15 were slightly 
more virulent than others, but the 
dilTercnces were neither great nor 
consistent enough to justify the con- 
clusion that they were genetically 
different — that is, different biotypes. 
The first conckisive evidence that some 
isolates were less virulent than others 
was obtained by studying a collection 
from Japan, which produced smaller 
pustules on some differential wheat 
varieties than any of the United States 
collections. All of the differential va- 
rieties except Khapli were susceptible, 
but some were distinctly less suscep- 
tible than to isolates from the collec- 
tions previously studied. The Japanese 



YEARBOOK Of AGRICULTURE 1953 


40 

rust was therefore designated as 1 5A, a 
distinct biotypc. 

Race 15 therefore included at least 
two biotyf)es, and it was suspected but 
not proved that there were still others. 
In 1937, however, an isolate was ob- 
tained that was distinctly more viru- 
lent on some differential wheat varie- 
ties than any previously studied, and 
it was therefore designated as 15B, a 
third biotype within race 1 5. 

Although clearly different genet- 
ically, the known differences between 
the biotype originally designated as 
race 15 and those subsequently found 
and designated cis 15A and 15B were 
not great enough to justify considering 
them as separate races. As i5)B ap- 
peared to be a potential menace to 
resistant varieties grown in northern 
United States, attempts were made to 
learn as much as possible about it. 
As it occurred only sparingly in nature, 
however, studies had to be limited to 
the very few isolates that could be ob- 
tained. Attempts were made to find 
varieties that would distinguish more 
clearly l)etween 15 and 15B. After 
extensive investigation, research work- 
ers found that a relatively new variety, 
Rival, and ar Australian hybrid, 
Kenya x Gular, w^ere fairly good 
differentials. Finally the new variety 
Lee (Hope x Timstein), released to 
growers by the Minnesota Agricultural 
Experiment Station and the Depart- 
ment o* Agriculture in 1951, proved 
resistant to 15 and susceptible to 15B, 
thus serving as a new indicator or 
differential variety for distinguishing 
clearly between 15 and 15B on the 
basis of resistance and susceptibility 
instead of on the le.ss satisfactory basis 
of degrees of susceptibility. That 
seemed like progress in understanding 
a complex situation, but the situation 
itself soon became more complex. 

In 1950 race 15B became wide- 
spread and prevalent in North 
America for the first time — at least 
since races of w'heat stem rust were 
recognized. Previously it had been 
found occasionally near barberry 
bushes, especially in the Eastern 


States; consequently it had not been 
possible to study many collections of 
15 or i^B, either because they did not 
exist or could not be found among 
the approximately r,ooo collections 
of wheat stem rust identified each year. 
But in 1950 there was abundant 
material because 15B caused a destruc- 
tive epidemic on previously resistant 
varieties in parts of Minnesota and the 
Dakotas and was isolated from 3 1 7 of 
the 810 samples of w'hcat rust obtained 
from 1 7 States. 

AH of the 3 1 7 isolates that w'erc iden- 
tified as 1 56 in 1950 produced essen- 
tially the same infection types on the 
12 differential varieties of wheat and 
on Lee wheat; otherwise they would 
not have been designated as 15B. 

There were some indications, how- * 
ever, that some of the isolates were not 
completely identical. Iliese results were 
obtained by the senior author and his 
colleagues at the Federal Rust Labora- 
tory, University Farm, St. Paul, Minn. 
Therefore 17 selected isolates were 
tested on 99 additional varieties of 
wheat and on 17 varieties of barley. 
Those tests — which were made by 
Rosendo Postigo, a graduate student 
at the University of Minnesota, witli 
the collaboration of Helen Hart and 
E, C. Stakman— “disclosed that 14 of 
the 1 7 isolates differed clearly in their 
effects on one or more of these varieties, 
some of which had been produced 
recently. It is evident, then, tliat bio- 
types of rust that seem to be identical 
because of their identical behavior on 
many varieties may prove to be differ- 
ent if additional biologic indicators can 
be found. The w^heat varieties are, of 
course, really biologic indicators or 
testers, and mu.st be depended on for 
distinguishing between rust biotypes, 
because the rust grows only on living 
plants and there is no known method 
of distinguishing between biotypes ex- 
cept by their effects on varieties of 
living plants. 

The experience wdth race 1 5 and 1 5B 
shows that recognition of differences 
between biotypes and between races 
depends on the availability of host vari- 



PROBLEMS OF VARIABILITY IN FUNO 


etics that react differently to them, and 
new varieties may serve as additional 
differentials. Conclusions regarding 
rust races therefore are limited by the 
varieties available at a given time; fur- 
thermore, as new races are continually 
being produced, it is necessary period- 
ically to revise the classification of 
races. 

Puccinia graminis is a good example of 
the complexity within the species, 
within the variety and even with- 
in races. There arc a great many bio- 
types, and their recognition and group- 
ing into races for practical purposes are 
subject to limitations of suitable differ- 
ential varieties at any given time and 
place. To recognize the differences be- 
tween all biotypes in existence would 
require that all varieties of wheat, bar- 
ley, and many grasses be inoculated 
under several combinations of environ- 
mental conditions with an adequate 
sample of rust collections from the 
world — and while that w'as being done, 
many new' biotypes probably would 
have been produced by mutation and 
hybridization. 

The problem of identifiying races is 
difficult not only because different bio- 
types may appear alike on many vari- 
eties and under many environmental 
conditions. It is difficult also becatise 
the infection types jiroduced by a 
single biotype on a single variety may 
vary widely with environmental con- 
ditions. As an example, Australian and 
American isolates of race 34 of P. gra- 
minis tritici differ in temperature rela- 
tions and in color, thus suggesting that 
different bio types may behave alike at 
certain temperatures but not at others. 
Certain races produce a type x infec- 
tion under some conditions but not 
under others. Thus the only known dif- 
ference between races 17 and 37 of P. 
graminis tritici is that the latter produces 
type X on Kubanka w'hile race 1 7 pro- 
duces tyf)c 3 to 4. Likewise, the differ- 
ence between races 36 and 125 is that 
the latter produces type x on Ku- 
banka; the difference between the 
races is genetic, but it is slight and vari- 
able enough to be obscured by differ- 


4 * 

ences in light and temperature. The 
same is true of some of the rac6s of oats 
stem rust, Puccinia graminis avenae. 

Despite the difficulties, it has been 
possible in the past to explore within 
the species Puccinia graminis sufficiently 
to explain differences in behavior of 
stem rust of wheat and oats in different 
times and places and to predict what 
is likely to happen in the future. Some 
races arc relatively stable genetically 
and therefore are consistent in be- 
havior historically and geographically; 
others are unstable and variable. 

Knowledge regarding the diversity 
within the species is incomplete. Were 
it possible to grow all stages of the rust 
on artificial media, it probably would 
be possible to recognize many more 
biotypes than is now possible. 

Although most plant scientists arc 
familiar with the life history of P. gram- 
inisy its relation to genetic studies de- 
serves emphasis. There are five stages: 
Telial, sporidial, pycnial, accial, and 
uredial. The mature black spores or 
teliospores are diploid, but reduction 
division occurs when they germinate, 
and the sporidia formed on the four- 
celled promycelia arc haploid and of 
two sex groups. The sporidia can infect 
only certain species of barberry (species 
oiBcrberis and Mahonid) on which they 
produce pycnia containing pycnio- 
spores, which are in reality specialized 
sex cells or gametes. 

Although the pycnia and pycnio- 
spores all look alike, they are of two 
kinds sexually, having been produced 
by sporidia of different sex. All the 
pycniospores produced within a single 
pycnium are of the same sex, so that 
pycniospores from pycnia of different 
sex must be brought together (in nature 
this is usually done by insects) in order 
to initiate the sexual Stage. This is 
therefore roughly analogous to cross- 
pollination in dioecious higher plants. 
The nuclei of opposite sex pair at the 
base of the aecium, divide simultane- 
ously, but do not fuse for a long time. 

From the “fertilized” pycnia are 
formed the cluster cups or aecia on the 
barberry, and in them are produced 



YEARBOOK OF AGRICULTURE 1953 


42 

the aeciospores, each with paired 
nuclei of opposite sex. The aeciospores 
cannot infect the barberry, but only 
appropriate grains and grasses on 
which the uredospores are produced 
in pustules, or uredia. The uredaspores 
contain paired nuclei also, and they 
can infect only grains and grasses, but 
this is the repeating stage of the fungus, 
and successive generations of uredo- 
spores can be produced indefinitely, 
provided susceptible kinds of grains or 
grasses are available and in vegetative 
condition. When conditions become 
unfavorable for the development of the 
uredial stage, the telial stage is formed. 
The young teliospores have paired 
nuclei, which fuse within the spore, 
and thus the final stage in the long 
sexual pj;pcess results in the formation 
of diploid nuclei. As the fused nuclei 
may differ in factoi*s for pathogenicity, 
segregation and recombination can re- 
sult in the production of new biotypes. 

The fact that the uredial stage of the 
rust is the only one that can be clonally 
propagated means that races must be 
identified in this stage, and, since it 
has paired nuclei of opposite sex that 
later fuse in the telial stage, a single 
heterozygous race may yield many 
different races as a result of segregation 
and recombination even if it is selfed. 
In reality so many biotypes often can 
be isolated from or near barberry that 
it is extremely difficult to group them 
into racc5, because many are so nearly 
alike and becau.se it often is hard to 
find varieties that show even the larger 
differences. 

The common com smut, Ustilago 
zeaey is suitable for studying biotypes 
within species. The smut spores 
(chlamydospores) result from the fusion 
of haploid nuclei. Accordingly the 
chlamydospore nucleus has a set of 
chromosomes from each parental 
line — ^that is, double the number (dip- 
loid) in each sex cell (gamete). On 
germination, the spore sends out a 
four-celled tube, the promycelium, 
like that produced by teliospores of 
stem rust. Each cell likewise produces 
. a small, sporelike body, known as a 


sporidium, which has a single nucleus 
with half as many chromosomes (hap- 
loid) as there are in the chlamydospore 
nucleus; this results from reduction 
division early in the process of ger- 
mination. The sporidia are therefore 
sex cells or gametes, like sperms and 
eggs, except that those of different sex 
cannot be distinguished by appear- 
ance; they look alike, but those of op- 
posite sex will conjugate (fuse) whUe 
those of the same sex will not. The 
test of sex, therefore, is behavior and 
not appearance. 

But these gametic sporidia are un- 
like most specialized sex cells because 
they can either multiply by ycastlike 
budding or germinate like spores and 
produce an extensive mycelium. This 
stage of the smut can be grown on or 
in sterilized nutrient media. Although 
only about 8 microns long (one three- 
thousandth of an inch), single sporidia 
can be picked up with a micromanip- 
ulator and placed on solid nutrient 
media in sterilized flasks; within about 
3 weeks colonies have formed, consist- 
ing of about 2 billion sporidia or a net- 
work of hyphae comprising a billion or 
more cells, all derived from a single, 
haploid cell. Such cultures arc called 
monosporidial lines and are a single 
bigtype, unless new biotypes are formed 
by mutation. There 'is no chance for 
segregation, because the original spor- 
idia themselves arc the product of seg- 
regation. The colonics produced from 
different sporidia often differ in many 
characters, such as size, color, and 
surface patterns. It is like looking at 
different kinds of forests from an air- 
plane, with each forest comprising one 
kind of tree. 

E. C. Stakman and L. J. Tyler at the 
University of Minnesota in 1934 
crossed two monosporidial lines with 
five clearly contrasting characters. By 
1952 many investigators there had 
produced about lo thousand different 
biotypes from recombinations and 
mutations among the descendants of 
the two original sporidia. These bio- 
types differ from each other in one or 
many major or minor characters, in- 



PROBLfMS OF VARIABILITY IN FUNOI 


eluding size, color, and topography of 
colonics, many other cultural charac- 
ters, sporidial or mycelial growth type, 
many physiologic characters, mutabil- 
ity, and sex and pathogenicity. Oc- 
casional monosporidial lines are dip- 
loid instead of haploid, but they are 
exceptional. 

Color of colonies among these bio- 
types ranges from aniline black through 
many shades and tints of purple, brown, 
pink, drab, and white. Innumerable 
patterns and combinations of topo- 
graphic patterns are superimposed on 
the profuse array of colors- - various 
kinds of concentric bands and ridges 
and many kinds of radial lines, folds, 
furrows, and ridges, some straight, 
some curving clockwise and some 
counterclockwise. There are many 
types of marginal growth. Some bio- 
types produce sporidia only. Others 
produce mycelium only. There also are 
many intermediate kinds, in which the 
growth type is determined by the 
amount of sugars in the medium. 

Temperature requirements differ 
and may affect relative survival in 
mixtures of biotypes. 

There are multiple sex groups and 
many gradations in pathogcnicity.There 
are wide differences in mutability, and 
it is becoming evident that in cultures 
of some of the most constant mono- 
sporidial lines there are mutants whose 
presence is masked under most con- 
ditions by the growth of the original 
lines. These masked mutants may 
persist indefinitely without betraying 
their presence unless special methods 
or media are used to detect them. 

Several significant facts have been 
obtained from investigations of Ustilago 
JZeae. There are thousands of haploid 
bioty]x;s within the species. Biotypes 
may differ from each other in one or 
more of all known characters, includ- 
ing pathogenicity. Differential media 
may be necessary to distinguish be- 
tween biotypes, just as differential 
varieties of crop plants are necessary 
to distinguish between physiologic 
races and biotypes of rusts. The asex- 
ually produced progeny of a single 


43 

*■ 

haploid sporidium, which should con- 
stitute a single biotype if there were 
no mutation, rarely do constitute a 
single biotype, even when the line 
appears pure and constant, because 
mutation occurs so commonly and 
many mutants may be masked. The 
virtual impossibility of guaranteeing 
genetic purity of the cultures suggests 
caution in interpreting results of 
experiments. Despite the vast diversity 
within the species, the characters of 
the spores (the chlamydospores) have 
been remarkably uniform, even though 
produced by thousands of different 
combinations of haploid lines. 

Possibly Ustilago ZfiM represents the 
extreme of intraspecific diversity, or 
possibly it is only better known than 
many other plant pathogens. Covered 
kernel smut of sorghum, Sphacelotheca 
sorghi, and the grass smuts, Sorosporium 
syntkerismae and Ustilago sphaerogena, 
also comprise many biotypes. Some 
smuts, however, such as loose smut of 
barley {Ustilago nuda)^ loose smut of 
wheat {U, tritici)^ loose smut of oats 
(U. avenae), and covered smut of 
barley {U. hordei) seem to be somewhat 
more homogeneous and constant. 

Investigations on rusts and smuts 
have been mutually complementary in 
explaining variability of pathogenic 
behavior. In the rusts only the binu- 
cleate uredial stage can be propagated 
clonally. In the smuts only the 
haplophase can be so propagated. 

In the smuts and rusts, three distinct 
phases correspond with the nuclear 
condition: The haplophase, in which 
the cells have nuclei with the reduced 
number of chromosomes, such as the 
sporidia of both groups and the pycnial 
stage in the rusts; the dicaryophase, in 
which two nuclei of opposite sex are 
paired in the cells but iupt fused; and 
the diplophase, in which the two 
nuclei of the dicaryophase have fused. 
The haplophase in the rusts (sporidia 
and pycnial stage) is restricted almost 
entirely to a sexual function. In some 
of the smuts, however, the sporidia 
(haplophase) can function both as 
propagative cells and as sex cells, but 



YIARBOOK OF AORICULTURI 1953 


the ability of the haplophasc to grow 
parasitically is limited. In both the 
smuts and rusts, the dicaryophase is 
the important parasitic phase, and it 
is important to realize that there are 
dicaryotic hybrids. This simply means 
that each cell can contain two nuclei 
of different sex and with different 
factors for pathogenicity and for other 
characters. The final hybridizing act 
is not complete until the two nuclei 
fuse to form the diplophase, which is 
restricted largely to the resting spores, 
the teliospores of the rusts and the 
chlamydospores of the smuts, except 
that diploid sporidial lines of smuts are 
encountered occasionally. 

The uredial stage of rusts can be 
propagated indefinitely on appropri- 
ate hosts; hence, repeated experiments 
can be made on the pathogenicity of 
dicaryotic hybrids, where minor dif- 
ferences in pathogenicity are percep- 
tible. In the smuts, on the other hand, 
the gametic haplophasc can be propa- 
gated saprophytically on the artificial 
media where conditions can be varied 
and controlled, but the pathogenic 
dicaryophase cannot be propagated 
clonally even on living plants. It 
comes to an end with the formation 
of diploid spores. Accordingly the 
progeny of a single uredo.spore in the 
rusts constitute a biotype, barring 
mutation; in the smuts the progeny of 
a single haploid sporidium constitute 
a biotype, barring mutation. Likewise 
the monosporidi^ diploid lines that 
occasionally appear in smuts are bio- 
types, barring mutation and segrega- 
tion. A physiologic race in the rusts, 
then, can he purified to a single bio- 
type, while in the smuts the most 
practicable way of determining the 
parasitism of so-called physiologic 
races is to inoculate differential vari- 
eties with successive crops of chlamydo- 
spores. 

The term physiologic race, then, does 
not have the same genetic connotation 
in the rusts and smuts. In many of 
the Fungi Imperfecti, which are im- 
perfectly known genetically, the only 
criterion of races must be behavior 


in culture and on host plants. As 
commonly used, therefore, the term 
physiologic race means a biotype or 
group of biotypes that can be distin- 
guished with reasonable certainty and 
facility by its physiologic characters, 
including pathogenicity, and, in some 
fungi, growth characters on artificial 
media. 

Under that definition, most species 
of plant pathogenic fungi comprise 
physiologic races. Some of the downy 
mildews (including Phytophthora m- 
Jestans^ which caases late blight of 
potato), some of the powdery mil- 
dews, and several of the wood-rotting 
fungi comprise many races. The spe- 
cies of Helminthospoiium that cause leaf 
spots, seedling blights, and root rots 
of small grains and corn all are com- 
plex in composition. Helminthosporium 
sativurriy H. gramimum^ H. tereSy H. 
carbonuniy and H. vicioriae comprise 
many races, whether judged by be- 
havior on artificial media or on host 
plants or both; H. sativum is known 
to be in the same category as Ustilago 
zeae with res]:)ect to diversity. Various 
species of Fusariumy such as those that 
cause flax wilt, melon wilt, tomato 
wilt, and head blight or scab of ce- 
reals, comprise many races. Colleto- 
trichum liniy which causes anthracnose 
of flax, C. lindemuthianumy the cause of 
bean anthracnose, Rhizoctonia solaniy 
which causes blights and rots of hun- 
dreds of kinds of plants, the apple 
.scab fungus, the fungus causing brown 
rot of stone fruits, and many others 
comprise numerous physiologic races. 

The determination of phenotypic 
variability of fungi — the temporary 
changes in appearance or behavior due 
to environment — is relatively easy 
when the fungi can be grown on 
nutrient media, as environmental fac- 
tors usually affect the fungus and not 
the medium. Simjlar determinations 
are more difficult when the fungus is 
growing on living plants, because en- 
vironment affects the fungus, the host 
plant, and the interactions between 
the two. 



PROBLEMS OF VARIABILITY IN FUNGI 


The phenotypic variability caused 
by nutrients, temperature, and other 
factors is often so great as to obscure 
genetic differences between biotypes 
or races when they arc determined 
partly by cultural characters and by 
pathogenicity. It is essential therefore 
to standardize media and other con- 
ditions with varying degrees of exact- 
ness, depending on the fungus and 
the objectives of the investigation. 
A primary requisite is to determine 
the limits of phenotypic variability of 
isolates before making conclusions re- 
garding genetic differences between 
them. 

Variability in growth type of com 
smut due to nutrition, temperature, 
and other factors can be wide or nar- 
row, depending on the biotype. An 
example: Mycelial patches or sectors 
in colonics of certain sporidial lines 
seemed to be mutants until it was 
found that subcultures from dissimilar 
sectors sometimes were identical. Phys- 
iologic and genetic experiments by 
M. F. Kemkarap, at the University of 
Minnesota, then showed that there are 
strict sporidial lines that cannot pro- 
duce mycelium, strict mycelial lines 
that do not produce the sporidia, and 
sporidial-mycelial lines that can pro- 
duce both sporidia and mycelium. The 
relative tendency toward sporidial or 
mycelial production in the latter group 
differs in different lines, but a high 
content of sugar in the nutrient me- 
dium favors production of sporidia and 
a low content of sugar favors formation 
of mycelium. Change from one type of 
growth to another may result from 
mutation also; hence it is always im- 
portant to find out whether the change 
is temporary and nonheritable or 
whether it is permanent and heritable. 

The same biotypes, then, may have 
several distinctly different phenotypes. 
Conversely, different biotypes may 
appear alike on certain media and not 
on others. Phenotypic differences may 
persist to some extent for one or more 
cultural generations; consequently the 
classification of groups of isolates 
should be made only ^ter they have 


45 

been grown for one or more genera- 
tions under identical conditions. 

Variability in development of fungus 
pathogens on living hosts can be stud- 
ied advantageously in some of the rust 
fungi because of the distinct infection 
types produced. From investigations 
by W. L. Waterhouse and associates at 
Sydney University, in Australia, Thor- 
v^dur Johnson and others at the Do- 
minion I.,aboratory of Plant Pathology, 
Winnipeg, and by Helen Hart and 
associates at the University of Min- 
nesota, it Is now known that tempera- 
ture and light can cause fundamental 
differences in the development of cer- 
tain races of stem rust on certain varie- 
ties of wheat and oats. Certain Kenya 
wheats and some hybrids, for example, 
are almost immune to race 15B of 
wheat stem rust at 65° F. and com- 
pletely susceptible at 85®. Conversely, 
race 34 develops best on some varieties 
of wheat at moderate temperatures 
and relatively low light intensity, and 
the infection type is depressed by high 
temperature and high light. The oats 
variety Hajira is resistant to known 
races of jP. graminis avenae below about 
80® but it is susceptible to race 6 at 85° 
and to races 7 and 8 at 90®. The impli- 
cations of phenotypic variability in the 
identification of races arc important, 
because a given rust race may produce 
infection type i on a host variety at 
temperatures as high as 75® and type 4 
at 85®. This is true of some race-variety 
combinations but not of others, and 
must therefore be studied for each 
combination. 

The term adaptation, as used here, 
means the capacity of a single biotype 
to acquire and transmit the ability to 
do something that it either could not 
do originally or could not do well. 
Thus, race 19 of wheat stem rust can 
produce only small pustules on Mar- 
quis wheat. It would be a case of 
adaptation if genetically pure race 19 
were grown on Marquis several suc- 
cessive rust generations and produced 
successively larger pustules until it 
acquired the ability to grow well on 



YEARBOOK OF AGRICULTURE 19S3 


46 

this variety, or at least better than it 
did originally. Likewise it would be 
adaptation if a single biotype of the 
com smut fungus {Ustilago Kfiai) were 
grown for several successive genera- 
tions on a medium containing arsenic 
and thus acquired the ability to toler- 
ate several times as much arsenic as it 
did originally. 

Between 1900 and 1910 the question 
of adaptation was studied by several 
investigators, who concluded that cer- 
tain rust fungi and powdery mildews 
could adapt themselves quickly to re- 
sistant varieties. 

Marshall Ward made many experi- 
ments with rust of bromegrasses 
{Puccinia dispersa) at Cambridge Uni- 
versity, England, and published a 
paper on adaptive parasitism in 
1903. He concluded that the bromc 
rust could develop the ability to grow' 
well on a resistant species of Bromus if 
grown on the resistant species for one 
or more successive rust generations. 
He also concluded that there were 
“bridging species” of Bromus that en- 
abled the rust to attack very resistant va- 
rieties; for example, the rust could grow 
well on variety A but not on variety C. 
If, then, a variety B could be found 
that was intermediate taxonomically 
between A and C, the rust could be 
grown on B and there it acquired the 
ability to infect C. Variety B, there- 
fore, was considered a bridge or 
“bridging host” between the susceptible 
A and the resistant C. 

Ward’s general conclusions were 
soon supported by results of experi- 
ments made by E. S. Salmon with 
powdery mildew (Erysiphe graminis)^ 
also at Cambridge; by those of E. M. 
Freeman and E. C. Johnson with 
Puccinia graminis at the University of 
Minnesota; and by experiments of 
I. B. Polc-Evans with stem rust of 
wheat in South Africa. Salmon con- 
cluded that the powdery mildew 
could attack injured plants of a nor- 
mally resistant species and thus ac- 
quire the ability to infect noninjured 
plante of that species. Pole-Evans 
applied the principle of bridging to 


the breeding of stem rust resistant 
varieties of wheat. He stated that some 
hybrids between resistant and sus- 
ceptible parents were more susceptible 
than the susceptible parent and even 
enabled the rust to increase its 
virulence for the resistant parent. 

In cooperative investigations be- 
tween the United States Department 
of Agriculture and the University of 
Minnesota, E. C. Stakman, F. J. 
Piemeisel, and M. N. Levine studied 
the possible adaptation of stem rust of 
wheat and other cereals and grasses 
and of powdery mildew of wheat and 
barley, but obtained no evidence that 
these fungi could adapt themselves 
to resistant varieties by any of the 
methods previously tried nor by any 
new methods that could be devised. 

In a paper published in 1918 they 
stated: “The facts given in this paper 
do not support the conclusions of 
previous workers that the patho- 
genicity of biologic forms is easily 
changed by host influence. . . . From 
the practical standpoint the constancy 
of biologic forms is of great impor- 
tance. Breeding for rust resistance can 
proceed with considerable assurance 
that the same rust will not adapt itself 
quickly to new varieties.” 

They also pointed out that dif- 
ferential hosts must be used to separate 
different biologic forms from mixtures 
before making experiments on adapta- 
tion; otherwise what appears to be 
adaptation may be merely the result 
of the selective effect of host plants on 
a mixed population of the pathogen. 
At that time, the now obsolete term 
“biologic form” was used to designate 
what are now called varieties of stem 
rust; races were just being dist'.ovcred 
within the tritici and other “biologic 
forms”; and eventually new facts led 
to new concepts and to changed 
terminology. 

These results seemed to prove that 
stem rust and powdery mildew of 
cereal grains did not change by 
adaptation. But the rusts and powdery 
mildews are obligate parasites; it 
has not been possible to grow them 



l»ltOBLEMS OF VARIABILITY IN FUNG 


on anything except living plants. 
Accordingly it was desirable to study 
possible adaptation or changes in 
virulence in fungi that can grow both 
in living host plants and on nutrient 
media — that is, both as parasites and 
as saprophytes. 

The results of extensive experiments 
made by J. J. Christensen and G. L. 
Schneider at the University of Minneso- 
ta from 1 946 to 1 950 with Helminthospori- 
um sativum, which causes leaf spots, foot 
rots, and root rots of barley, wheal, 
and other cereals and grasses, support 
the view that genetically pure lines of 
fungi do not change their virulence 
easily. The i.solate of H. sativum they 
studied had been grown on artificial 
nutrient media for 28 years and had 
been purified bv making many suc- 
cessive single-spore isolations. Wheat 
plants were inoculated with single 
spores; when new spores were pro- 
duced, single ones were again picked 
and inoculated into plants with a fine 
needle. This process was repeated ten 
times in succession, and there never 
was any evidence of change in viru- 
lence. This extreme refinement of 
method was necessary because H. 
sativum mutates rather freely, produc- 
ing many mutants that are less patho- 
genic and a few that are more patho- 
genic than the parental line. Some of 
the less patliogenic lines grow very 
rapidly and therefore tend to over- 
grow the original parental line. Unless 
the fungus is grown under conditions 
that permit the recognition of mutants 
and their separation from the parental 
line, an initially pure isolate may .soon 
comprise a diverse mixture of mutant 
lines, and the original line may even 
have been lost in the process of making 
periodic -transfers to new tubes of nu- 
trient media. Obviously, then, an 
isolate of H. sativum derived from a 
single spore may change in virulence, 
but the change is due to the production 
of new biotypes resulting from muta- 
tion and not to a change in virulence 
of the original biotype. 

It would be easy to conclude that 
some of the smut fungi increased in 


47 

virulence for resistant varieties as a 
result of succc.ssive passages through 
those varieties if the intraspecific com- 
plexity of smut species were not known. 
Many investigators, notably W. A. R. 
Dillon-Weston in England, tested the 
resistance of varieties of wheat, barley, 
oats, and other crop plants to various 
smuts. When first inoculated with a 
collection of smut spores (chlamydo- 
spores), very little smut develops on 
some varieties. If the smut ffom a 
resistant variety is used to inoculate 
plants of the same variety, heavy in- 
fection may result, becau.'se nonpurified 
collections of smut spores are likely to 
comprise many parasitic races that can 
be separated from each other by cer- 
tain varieties. A.ssuming that three 
wheat varieties. A, B, and C, are 

inoculated with a smut collection con- 
taining races i, 2, and 3 in the ratio 
of 90:9:1 and as.sumiiig that all three 
races infect variety A normally, that 
race 2 infects B but not C, and that 
race 3 infects C and not B, the per- 
centages of infection theoretically 
should be the following: 

A B C 

Percentage of infection . . 100 9 t 

Races 2, 3 2 3 

Percentage of each race . 90:9:1 100 

Obviously, if plants of variety A are 
then inoculated with the smut pro- 
duced on A, spores of all three races 
w'ill again be produced. But if variety 
B is inoculated with spores from B, 
100 percent of them will be of race 2; 
and if variety C is inoculated with 
spores from C, 100 percent of the 
spores will be of race 3, the only one 
in the original mixture that can attack 
C. Thus, varieties B and C are bio- 
logic indicators, or differential vari- 
eties, that make it possiWe to find out 
not only which races were presenf<*iBpt^ 
the original smut collection but itrient 
the relative percentages of each. More- ’ 
over, the apparent adaptation of the 
smut to varieties B and C is not 
adaptation at all but is due to .selection 
of the races that can attack those 
varieties out of a mixture in which 



YEARBOOK OF AGRICULTURE 1953 


48 

they were present in small amounts 
only. 

Even after races of smuts have been 
isolated and purified as we have de- 
scribed, the degree and permanence 
of purity are only relative because 
the spores are not always exactly 
alike. And, even if they were, the 
chlamydospores of smuts are the result 
of sexual fusions. When the spores 
germinate, segregation and recombi- 
nation usually occur, so that the prog- 
eny of a single spore might be diverse 
genetically. For practical purposes, 
physiologic races of smuts are usually 
considered to be collections of chlamy- 
dospores that behave with relative 
constancy in successive generations. 
Because of sexual recombinations and 
mutations within the races, however, 
they often comprise many biotypes, 
usually closely enough related so that 
the race behaves fairly consistently. 
The degree of refinement commonly 
practiced in classifying smut races 
usually suffices for determining the rel- 
ative resistance of crop varieties, but 
certainly not for studies on adapta- 
tion or other physiologic and genetic 
phenomena. It suffices for practical 
procedures but not for scientific under- 
standing. 

There are several other reasons why 
races or biotypes of fungi may appear 
to have changed, when in reality 
there has been a change in the kinds 
of biotypes. It has been pointed out 
in the discussion of Helminthosporium 
sativum that apparent changes in a 
monosporous line may be due to the 
production of new biotypes by muta- 
tion, so that the culture soon com- 
prises not only the original biotype 
but several others in addition; it be- 
comes a mixed or heterogeneous popu- 
lation instead of a pure or homogene- 
^'^ne. That the original bio type may 
1 ^ ‘ been shown by many 

'CTperiments. And even if it is not, 
the behavior of mixtures of biotypes 
cannot be predicted. 

Different races or biotypes of path- 
ogenic fiingi may not survive equally 
well in mixtures. This is to be expected 


if the environmental conditions favor 
one race more than the others. But it is 
sometimes true when conditions, as far 
as can be determined, are equally favor- 
able to the different races. Extensive 
studies of the relative survival races 
of wheat stem rust have been made at 
the University of Minnesota and at 
Sydney University, and typical results 
were published by I. W. Watson in 
1942 and by W. Q. Loegering in 1951. 
As the principle is more important 
than the details, which must be ascer- 
tained for each individual mixture of 
each fungus, a few examples are given 
to illustrate differential survival of crop 
plants and of fungi. 

H. V. Harlan and Mary L. Martini 
of the Department of Agriculture pub- 
lished in 1938 the results of experi- 
ments on the relative survival of barley 
varieties in a mixture grown in succes- 
sive years. H. H. Lande and A. F. 
Swanson made similar experiments on 
winter wheats in Kansas and pub- 
lished their results in 1942. The exper- 
iments with barley and winter wheats 
established the important principle 
that certain varieties persisted better 
than others in mixtures in which the 
relative percentage of each variety was 
determined in the first year and in each 
successive year, when seed was taken 
each year and used for the next year’s 
planting. The Darwinian principles of 
competition, natural selection, and 
survival of the fittest applied to these 
mixtures of varieties of barley and of 
wheat. Watson and Loegering tested 
the validity of the same principle on 
mixtures of races of wheat stem rust. 

Loegering prepared an approxi- 
mately 50:50 mixture of races 17 and 
1 9 of wheat stem rust and grew it for 
six successive uredospore generations 
on Litde Club, Fulcaster, and Min- 
dum wheats, all of which appeared to 
be equally susceptible to the two races. 
In each generation the mixture was 
tested on Marquis wheat, on which 
race 17 produces type 4 and 19 pro- 
duces type 2 infection, thus making it 
possible to determine the percentages 
of each race in the mixture. On Min- 



PROBLiMS OP VARIABILITY IN FUNOI 


dum the two races persisted almost 
equally well, although race 17 per- 
sisted slightly better than 19. But on 
Little Club and Fulcaster the percent- 
age of race 17 increased rapidly and 
that of 19 decreased correspondingly. 
In the second generation more than ^ 
percent of the rust was race 1 7 and less 
than 20 percent was race 19, and with- 
in 6 generations or fewer all or almost 
all of the rust was race 17 and none or 
almost none was race 19. In similar 
experiments with races 17 and 56 on 
Geres, Little Club, and Fulcaster 
wheats, all apparently equally suscep- 
tible to the two races, race 1 7 survived 
only slightly better than 56 on Ceres, 
but on the other two varieties it sur- 
vived far better than race 56, which 
had disappeared completely or almost 
completely at the end of seven genera- 
tions. These results are being fully con- 
firmed by James W. Broyles at the 
University of Minnesota, who is using 
a yellow-spore biotype of race 1 1 of 
wheat stem rust in mixtures with other 
races. As the pustules produced by race 
1 1 are yellow and those produced by 
other races arc brick red, the percent- 
ages of race 1 1 can he determined with- 
out the necessity of inoculating special 
differential varieties. Many experi- 
ments have been made to find out why 
some races of wheat stem rust persist 
better than others in mixtures, but no 
obvious reasons have been discovered. 

Mixtures of other fungi may behave 
like those of wheat stem rust, which 
have been used as an example because 
of the relative ease and precision of 
identification. Paul E. Hoppe at the 
University of Wisconsin inoculated 
corn with equally pathogenic strains 
of Diplodia which causes car rot 
and stalk rot, and the antagonism 
between strains was so great that only 
one survived. Similarly V. F. Tapke 
made experiments with different 
races of covered smut of barley and 
observed that there was differential 
survival of races. 

It appeared for a numl>cr of years 
that Phytophthora infestans, the fungus 
that causes late blight of potatoes, 


49 

could adapt itself to redstant vari- 
eties, as Donald Reddick and associ- 
ates, in their attempts to develop 
blight-resistant varieties in New York, 
ob^rved that resistant varieties might 
be slightly infected when first inocu- 
lated with certain races of the blight 
fungus, but became more severely in- 
fected when successive inoculations 
were made with the blight fungus 
from the same variety or certain 
others. Helena L. G. DeBniyn made 
similar experiments in the Nether- 
lands and confirmed the results of 
Reddick and others. 

H. D. Thurston and C. J. Eide, at 
the University of Minnesota, have 
obtained results that indicate that the 
apparent adaptation of the potato 
blight fungus is due, in some cases at 
least, to the selective effect of vari- 
eties on a mixed population of bio- 
types. M an example, two races of 
the blight fungus infect Irish Gobbler 
equally well, but one attacks Cherokee 
and the other does not. When these 
races were grown in mixture on Irish 
Gobbler, the Cherokee race decrea.scd 
rapidly in prevalence but did not 
di5ap]:>ear entirely; consequently, when 
the Cherokee variety is inoculated 
with the blight fungus from the Cob- 
bler variety the infection is very light, 
because so small a proportion of the 
racial mixture can attack it. But the 
blight that docs develop is caused by 
the Cherokee race, and when this is 
used to inoculate Cherokee again, 
there Hs abundant infection because 
all of the inoculum is potentially 
effective. 

The present authors and some of 
their associates began a comprehensive 
investigation of adaptation in 1935. 
As they had not been able to obtain 
evidence for adaptive changes in para- 
sitism, they investigated the possibility 
of adaptation to chemicals in nutrient 
media. Certain protozoa and bacteria 
were reputed to develop the ability 
to adapt themselves to deleterious 
chemicals; therefore studies were made 
independently by three groups of in- 
vestigators to find out if diis wer e true 



YBAKBOOK Of AQBICULTVBB 1953 


30 

also of fungL The resuJts were not 
uniform. In some instances mutation 
clearly accounted for what could have 
been considered adaptation. In other 
instances visible mutation did not 
account for the results. The results 
that were obtained by the three groups 
are given below. 

J. J. Christensen and several as- 
sociates grew the asexual {Fusarium) 
stage of Gibberella zeae^ which para- 
sitizes corn and causes head blight of 
wheat and barley, on nutrient media 
containing, respectively, ethyl mercury 
phosphate, a widely used seed dis- 
infectant; mercury bichloride; and 
malachite green in concentrations 
that dwarfed the growth of the fungus. 
The fungus produced numerous clearly 
visible mutants on each medium. Some 
grew more poorly than the parent on 
the special media. Some grew about 
equally well. Some made from 5 to 
1 5 times as much growth as the parent. 
Some of the mutants, in fact, grew so 
much faster on the mercury and 
malachite green media that they soon 
overgrew the original line completely. 
The relative ability of the original 
line and its mutants to tolerate these 
chemicals persisted through many 
generations. In the experiments there 
was no increase in ability to tolerate 
the chemicals tried, except as a result 
of visible mutations, which might 
easily have been overlooked if special 
methods had not been used to detect 
them. 

E. O. Mader and C. L. Schneider 
grew the asexual, or conidial, stage 
{Monilia) of Sclerotinia jmcticola^ the 
cause of brown rot of peaches, plums, 
and similar fruits, on nutrient media 
containing injurious amounts of copper 
sulfate. Some of their results were 
comparable with those of Christensen: 
Mutants were produced that grew 
much better than the parent on the 
copper media and they differed also 
in appearance. The increased toler- 
ance for copper was persistent and 
constant in the mutant lines. But the 
fungus also icquired increasing toler- 
ance for copper sulfate, without visible 


mutation, when grown in successive 
transfer generations on copper-con- 
taining media. When the fungus was 
grown for successive generations on 
copper-free media, however, it lost 
the acquired ability. 

The third series of Minnesota experi- 
ments were made by Coyt Wilson, 
Frank Stevenson, Donald Munnecke, 
J. M. Daly, Elisa Hirschhom, and 
£. C. Stakman. This group studied the 
adaptation of corn smut {Ustilago z^ae) 
to arsenic. Tliey grew mutable and 
relatively nonmutable monosporidial 
lines on nutrient media containing 
2,400 parts per million of calcium 
arsenite. All lines grew poorly at 
first, but after having been transferred 
successively for about 10 generations 
to increasing concent rations of arsenic, 
all lines eventually grew as well on 
media containing 12,000 parts per 
million of calcium arsenite as they had 
originally grown on 2,400 parts per 
million, and some lines even grew on 
14,000 parts per million. When grown 
for five successive generations or more 
on arsenic-free media, the arsenic- 
adapted lines lost their acquired 
ability. Numerous mutants appeared 
in the mutable line growing on arsenic 
media. Some appeared in the relatively 
stable line, but only two grew slightly 
better than the parents on arsenic. 
It was the consensus cf this group of 
investigators, some of whom worked 
independently at different times with 
different lines and with different 
methods, that perceptible mutations 
did not account for the results. All 
smut lines that were tried, including 
the least mutable ones among thou- 
sands that were studied by Stakman 
and many associates for more than 20 
years, increased their ability to grow 
on arsenic. It is possible that there 
had been unseen mutation in physio- 
logic characters, but, if so, these 
mutants escaped detection. 

From extensive experiments with 
several fungi, at Michigan State Col- 
lege about 25 years ago, P. D. Caldis 
and G. H. Coons concluded that white 
variants that appeared in single-spore 



PROBLEMS OF VARIABUITY IN FUNCI 


cultures of several fungi were ‘^semi- 
permanent” variations, which differed 
from the parent form somatically 
rather than genetically and were com- 
parable with the “dauermodifications” 
described for paramecia by Victor 
Jollos. The genetic explanation for this 
type of semipermanent change is still 
lacking. It is commonly observed that 
there may be what is sometimes called 
a “hangover” effect when fungi are 
grown for some time on one kind of 
nutrient medium and then transferred 
to another kind. This may be accounted 
for partly by the production of adaptive 
enzymes or other chemical substances, 
but the exact explanation cannot be 
given for many of these temporary 
changes. 

Although it cannot be asserted cat- 
egorically that parasitic fungi never 
adapt themselves to resistant varieties 
of plants, there is convincing evidence 
that apparent adaptation often is due 
to the selective effect of crop varieties 
on a mixed population of biotypes, 
cither because the isolate or line of the 
fungus was genetically diverse at the 
beginning, even though it appeared 
pure by all tests then available, or be- 
cause it became mixed as a result of in- 
conspicuous mutations. The apparent 
loss of virulence in a single bio type fre- 
quently unquestionably is due to the 
production of mutants that look like the 
original biotype but have lost genetic 
factors for virulence. The apparent ac- 
quisition of ability to tolerate injurious 
chemicals certainly is due to mutation 
in some instances but appears to occur 
independently of mutation in others. 
This is still an unsolved problem and 
one that requires intensive research. 
The difficulty of obtaining and main- 
taining genetically pure cultures of 
pathogenic fungi cannot be overem- 
pha.sized in research of this kind. 

Many pathogenic fungi mutate 
abundantly on nutrient media. There is 
evidence that mutation occurs also on 
host plants in nature. The frequency of 
mutation differs greatly with the 
species, with the line or biotype within 


51 

a species, and with environmental con- 
ditions, including some mutagenic 
agents. In Ustilap z/^ae (corn smut), 
Puccinia gramims (stem rust), and Ven- 
turia inaequalis (apple scab), the factors 
for mutant characters have been shown 
to be inherited through the sexual 
stage. Factors for mutability in 17. zeae 
are inherited just as are factors for 
other characters. 

Mutation has been observed most 
commonly in cultures on solid nutrient 
media, where the mutants often appear 
as conspicuous sectors or patches in the 
colony. Many mutants remain unob- 
served, either because the medium on 
which they appear is not suitable for 
their growth, or because mutation in 
physiologic characters and in patho- 
genicity may occur v/ithout change in 
color, growth type, or other visible 
characters or because mutants may be 
obscured by the growth of the parental 
line. 

Mutants of obligate parasites, such 
as rusts, must manifest themselves on 
living hosts and often are not easily 
detected unless they cause conspicu- 
ously different infection types or arc 
different in color. 

Mutant factors usually are recessive. 
The parasitic stages of rusts arc mostly 
dicaryotic, and mutation for a single 
factor in the dicaryophasc can remain 
unexpressed until the factors are 
brought together in a homozygous 
condition in a new dicaryotic hybrid. 
'Fhe mutant characters can be ex- 
pressed immediately in the dicaryo- 
phase if the rust is in a heterozygous 
condition at the respective loci; or if 
a double mutation occurs, once for 
each locus in the two conjugate nuclei. 
A mutation, even if not expressed im- 
mediately in the dicaryophasc, may 
be expressed after recombination and 
segregation of factors during the sexual 
stage. Certain types of mutation, such 
as albinism, can be observed easily in 
haploid pycnia. 

Mutation for cultural charac- 
ters is common in all classes of fungi, 
but the frequency of mutation differs 



YEAIBOOK OF AGRICULTURE 1953 


52 

greatly in different species of the same 
genus and in different lines or biotypes 
within the species. Among the smuts, 
as examples, mutation is relatively in- 
frequent in Ustilago koUeri, covered smut 
of oats; U. avenae^ loose smut of oats; 
and Urocystis occulta^ stem smut of rye. 
On the other hand, it is extremely 
common in U. Zfioe, common corn 
smut r Sphacelotkeca sorghi, covered ker- 
nel smut of sorghum; and Sorosporium 
reilianum^ head smut of sorghum and 
com. 

The relative mutability of different 
biotypes of Ustilago Z/eae, according to 
Stalman and others, is due to heritable 
genetic factors. There is clear-cut seg- 
regation for mutability and constancy 
in some crosses between mutable and 
constant lines, but in other crosses 
segregates have many degrees of mu- 
tability. Mutability and constancy, re- 
spectively, were increased by crossing 
mutable x mutable lines and constant 
X constant lines, in extensive experi- 
ments made by Stakman and others. 
These investigators isolated the four 
primary sporidia (gametic segregates) 
from the promyceiium of one of the 
chlamydospores resulting from a mu- 
table X constant cross. Twenty -five (24 
in one case) single sporidia were then 
isolated from the progeny of each of 
the four sporidia, and the resulting 
lines were grown on nutrient agar in 
duplicate flasks. In the 100 colonies 
derived from sporidia i and 2 there 
were no mutants, but there were 360 
in the 98 colonies derived from sporidia 
3 and 4. 

Little is known about the numerical 
frequency of mutation in pathogenic 
fungi, but it falls well within the range 
reported for higher plants and for in- 
sects. Twenty or more sectors may 
appear in a single colony of a fungus 
growing on nutrient agar, but the 
colony may comprise millions of 
spores and hyphal cells so that the 
actual rate of mutation may not be so 
high as it seems. Christensen and 
Schneider calculated the frequency of 
mutation, based on the number of 
spores in colonics of certain lines of 


Helminthosporium sativum on nutrient 
agar, as from 1 12,400 to i :20,ooo, de- 
pending on conditions; and on living 
plants the rate was approximately 
1:2,900. S. P. Chilton, G. B. Lucas, 
and C. W. Edgerton, at the University 
of Louisiana, have concluded that 
certain factors in Glomerella sp. mutate 
at the rate of about 1:1,700. Studies 
now being made at Minnesota indi- 
cate that rate of mutation in some 
biotypes of Ustilago zyae is much higher 
than in H. sativum. In one biotype ol 
U, zeae^ 0.8 percent of the sporidia 
derived from a single sporidium were 
different genetically from the original 
sporidium. Thus, in a flask of liquid 
nutrient broth there were about 10 
billion sporidia, all derived from one 
original sporidium; 80 million of the 
10 billion were genetically different 
from the original. There had been at 
least five different kinds of mutations 
based on the character of color alone. 

Mutation may be important in the 
plant rusts even though the rate of 
mutation appears to be low, because 
there are at least 50,000 billion uredo- 
spores of stem rust on an acre of 
moderately rusted wheat, and even 
a few mutants per acre could multi- 
ply so rapidly under favorable con- 
ditions as to produce an extremely 
numerous population in one growing 
season. 

Mutation in fungi can occur for 
most physiological and morphological 
characters. The change may be in 
one or more characters and in many 
degrees of magnitude. Mutation is 
common for cultural characters, in- 
cluding color, topography, consist- 
ency of colonies, direction of growth, 
nature of margin, zonation, rate of 
growth, type of growth, and amount 
of sporulation. There is mutation for 
physiological characters (such as en- 
zyme production), reaction to known 
chemicals and to toxic substances, 
in temperature requirements, and in 
tendency to mutate. There often is 
mutation for morphological characters, 
principally size, shape, and color of 



PIOBLEMS OF VARIAilllTY IN FUNGI 


spores, fruiting bodies, and resting 
bodies. Mutation in sexual vigor and 
in pathogenicity also is common. 

Mutation in Ustilago Helmin- 
thospofivm sativum^ and many other 
fungi has been studied at the Uni- 
versity of Minnesota continuously for 
more than 30 years, and many thou- 
sands of mutants have been isolated 
and studied. The number and kinds 
of mutants produced by U, zitat and 
H, sativum are indefinite. Mutants may 
differ widely or very slighdy from 
their parents. As one example, a 
series of mutants from a single brown 
monosporidial line of U, z^ae ranged 
from near black through a score of 
tints to colorless. From another line, 
a white mutant was isolated from a 
pigmented line of U. ZJfcif, and this 
white mutant in turn produced white 
mutants, which in turn produced ad- 
ditional ones- The original mutant 
was then crossed with certain pig- 
mented lines to find out its breeding 
behavior, and all of the while .segre- 
gates from the cross were isolated and 
kept. In this way, 98 clearly distinct 
white lines, mutants and segregates, 
were obtained, which comprised a 
number of sex groups. Some of the 
numerous white x white crosses pro- 
duced large galls, but none produced 
chlamydospores. When some of the 
white lines were crossed with pig- 
mented lines, however, both galls and 
chlamydospores were produced. Evi- 
dently, therefore, the original white 
mutant had lost factors for color pro- 
duction and for nuclear fusion and con- 
sequent production of chlamydospores, 
although the nuclei of opposite sex 
in some of its progeny had the neces- 
sary factors for pairing. Certain of 
the white x white crosses therefore 
had the necessary factors for pairing 
of nuclei and for pathogenicity, but 
not for the final stage in the sexual 
process; hence there had been a loss of 
some but not all of the factors for sex. 
Although mutation in factors for sex 
and pathogenicity are common in 
smuts, complete sex reversal has not 
been reported. 


53 

It is harder to observe mutation for 
pathogenicity than for cultural char- 
acters. When it docs occur, however, 
there usually is partial or complete loss 
in pathogenicity and only occasionally 
is there a gain. This is conspicuously 
true of if. sativum and of U. zeae^ prob- 
ably the most thoroughly studied in 
this respect. When successive mutants 
of U. z^ae are isolated (that is, mutant 
from mutant from mutant and so on), 
a clearly descending scries from strongly 
pathogenic to nonpathogenic is likely 
to result. Occasional mutants, how- 
ever, are decidedly and consistendy 
more pathogenic than the parental 
line. This is true also of other fungi. 

Color mutations are not uncommon 
in rusts, and at least two such recorded 
cases also involved changes in patho- 
genicity. There is evidence that uredo- 
spores of orange and white races of 
Puccinia graminis are destroyed more 
easily by ultraviolet light than those 
with the normal darker color. This 
may partly account for the relatively 
poor survival ability and infrequent 
occurrence of the abnormally light- 
colored races in nature. 

Mutatioii in pathogenicity occurs 
occasionally in rusts and it probably is 
fairly common, although minor muta- 
tions arc hard to detect. In cooperative 
investigations between the Department 
of Agriculture and the University of 
Minnesota, E. C. Stakman, M. N. 
Levine, and R. U. Cotter studied four 
distinct mutations for pathogenicity in 
race i oi Puccinia graminis tritici^ and two 
of the mutants differed so much from 
anything previously described that 
they were designated as new races, 
numbers 60 and 68. Margaret Newton 
and Dr. T. Johnson, at the Dominion 
Laboratory of Plant Pathology, re- 
ported two separate mutations for 
pathogenicity in Puccinia graminis, one 
in the variety tritici and the other in the 
variety avenae. Mutation for patho- 
genicity also has been reported for 
Puccinia hordti, leaf rust of barley; P, 
glumarum, stripe rust of wheat and other 
cereals; and A rubigo-vera tritici^ orange 
leaf rust of wheat. Some of these rust 



YEARBOOK OF AGRICULTURB 1953 


54 

mutants were less virulent than their 
parents on some varieties, but some 
were more virulent than their parents 
on some varieties of host plants. 

In some fungi the number of visible 
mutants produced differs on different 
media. Whether this is always due to 
different rates of mutation or to dif- 
ferences in the conspicuousness of the 
mutants on different media is not 
known in all instances. Certain sugars, 
potassium salts, uranium and po- 
lonium salts, salts of some of the heavy 
metals, and some other chemicals 
generally increase the number of 
visible mutants. Either high or low 
temperature may also increase the 
number, depending on the species or 
biotype of the fungus. Ultraviolet light, 
radiations, and some of the bacterial 
products also are mutagenic. 

J. J. Christensen observed that mer- 
cury bichloride, ethyl mercury phos- 
phate, and malachite green in nutrient 
media were decidedly mutagenic to 
Gibberella ztae-, Helminthosporium sativum^ 
H, carbonum, Fusarium moniliforme^ and 
Colletotrichum Urn, IThe most significant 
fact derived from these experiments 
has been discussed in the section on 
adaptation, where it was pointed out 
that some of the mutants grew from 5 
to 1 5 times as fast as the parental lines 
on these chemicals. Christensen and 
F. R. Davies also found that bactcria- 
staled media caused very abundant 
mutation in monosporous lines of H, 
sativum. Among the mutants there were 
zero, plus, and minus deviations from 
the original lines in tolerance for the 
bacterial products, in pathogenicity, 
in rate of growth, and in many cultural 
characters. 

M. L. Gattani, E. C. Siakman, J. M. 
Daley, Shih I. Lu, and J. B. Rowell, at 
Minnesota , increased the rate of muta- 
tion in haploid and diploid lines of 
Ustilago zeae by adding uranyl nitrate, 
at the rate of about one gram per liter, 
to potato-dextrose agar. Most mu- 
tants grew as well as or better than the 
parental lines on the mutagenic me- 
dium. Lu studied 13 characters in 198 


uranium-induced mutants from a mon« 
osporidial haploid line of U, zfioe and 
summated algebraically the zero, plus, 
and minus deviations for each of the 13 
characters for each mutant. About 20 
percent had a net minus deviation, 20 
percent a plus deviation, and 60 per- 
cent a zero deviation, although all 
were, of course, different from the 
parent in one or more characters. 

Thus, deviation would be zero if a 
mutant exceeded the parent in size of 
colonies and in two other characters, 
had lost some factors for color and for 
two other characters, but was like the 
parent in the other seven characters. 
There would be a plus deviation if the 
mutant exceeded the parent in two 
characters, had lost in one, and re- 
mained tlie same in 10, and there 
would be a minus deviation if the plus 
and minus numbers were reversed. In 
similar studies on a solopathogenic 
(diploid) line, 10 percent of the mu- 
tants had a net minus deviation, 30 
percent plus, and 60 percent were zero. 
In similar experiments I. Wahl, also at 
Minnesota, produced mutants of the 
common mushroom, Agaricus campestris, 
that were much more vigorous than 
the original line and that produced 
mushrooms of more desirable color. 

Uranyl nitrate is not equally muta- 
genic to all fungi, being effective in 
about 10 percent of 194 species and 
lines studied by E. C. Stakman, J. B. 
Row^ell, How'ard Ehrlich, and others 
in an investigation made cooperatively 
at the University of Minnesota by the 
United States Atomic Energy Commis- 
sion and the University. Polonium 
salts have been found more effective 
than uranium salts as mutagenic agents 
in the investigations. 

From the studies of Ustilago zeae it is 
evident that even the most constant 
lines may contain mutants that may 
be unobser\'cd in many serial transfers 
on nutrient media and become visible 
only under special conditions or when 
dilution plates are made from shake- 
cultures in liquid media. This fact has 
many obviously important implica- 
tions. Although there are many known 



PIOBIEMS OF VAIIAeiLITY IN FUN01 


mutations in physiologic characters, 
there undoubtedly are many more that 
are not detected because there are no 
visible evidences of their existence in 
ordinary cultures. Several mutants 
with decreased pathogenicity may be 
present in a flask culture of a mono- 
sporidial line of JJ. zfioe containing 
about 10 billion cells. This could result 
in lowered patliogenicity of a line that 
originally comprised a single biotype, 
but a tremendous amount of work 
would be required to isolate the less 
virulent mutants if they were slow 
growing and like the original line in 
all observable characters except patho- 
genicity; if the less pathogenic mutants 
were exceptionally fast growers on arti- 
ficial media, they would tend to pre- 
dominate in the culture; and the path- 
ogenicity of the original line would 
appear to have been lost, when actu- 
ally the line itself had been suppressed 
or lost. There is abundant evidence 
that this is fact, not mere theory. 

H, sativum and U. zjsae have been dis- 
cussed so fully merely because they 
have been studied extensively and con- 
tinuously by many investigators at 
Minnesota, under the general super- 
vision of the writers, for about a quar- 
ter of a century, and because evidence 
is increasing that the principles derived 
from them apply in varying degrees to 
many plant pathogens. Above all, it is 
noteworthy that in f/. where single, 
haploid sporidia can be isolated and 
propagated ascxually, thus theoreti- 
cally establishing single-biotype cul- 
tures, it is extremely difficult, because 
of visible or invisible mutations, to 
maintain the genetic purity even of the 
apparently least-mutable monosporid- 
ial lines. At best, purity appears to be 
relative only. 

Stability among mutants differs 
greatly: Many mutants continue mu- 
tating indefinitely. Others are rela- 
tively stable. There are many inter- 
mediate degrees of mutability. Many 
mutants of Helminthosporium sativum^ H. 

H. ewhonum^ Gibberella and 
Fusarium Uni have been grown side by 


55 

side with their parental lines for many 
years and have retained their distinc- 
tive characters. Some have been grown 
on many kinds of media several years, 
and have retained their distinctive cul- 
tural characters. Many mutant lines of 
Ustilago zfiae have been grown under a 
wide range of conditions for many 
years without perceptible change, but 
others continue to mutate abundantly. 

G. S. Holton, in longtime experiments 
started at the University of Minnesota 
and continued under State and Federal 
cooperation at Washington State Col- 
lege, maintained an albino mutant of 
covered smut of oats, Ustilago kolleri, 
through many asexual and sexual gen- 
erations during the past 1 8 years. 

G. W. Keitt and M. H. Langford, at 
the University of Wisconsin, passed 
three haploid lines of Venturia inaequalis 
through the leaves of the McIntosh 
apples four successive times without 
perceptible change in cultural charac- 
ters. J. J. Christensen obtained similar 
results by passing mutants of Diplodia 
zeae^ Gibberella z^ae^ Helminthosporium 
sativum, and //. oryzof through their 
respective hosts, although in one case 
the cultural characters of a mutant of 

H. sativum were changed slightly. Mu- 
tant characters of Ustilago z^ae have 
persisted through the sexual stage on 
corn plants, even though they some- 
times were recombined with other 
characters. 

Heterocaryosk* means that condi- 
tion in which the hyphae, or individual 
cells of the hyphae, contain nuclei with 
different genetic factors. Heterocaryo- 
sis can come about independently of 
sex. From extensive investigations by 
H. N. Hansen and W. C. Snyder at the 
University of California, it appears 
that heterocaryosis is common in many 
of the fungi in which the sexual stage ii 
hot known, the Fungi Imperfecti. This 
condition can result from ordinary 
fusions between hyphae of different 
kinds or lines of fungi, and would be 
roughly comparable with natural grafts 
in higher plants. 

Sydney Dickinson, in experiments 



YEARBOOK OF AGRICULTURE 1953 


56 


at the University of Minnesota, grew 
a red Fusarium and a white one side 
by side, watched some of the hyphae 
fuse, and then cut off hyphal tips pro- 
duced by the fused hyphae and trans- 
planted them to nutrient agar. Some 
of the hyphal tips produced pink col- 
onies, all of which, however, later sep- 
arated into red and white. It appears, 
therefore, that nuclei from the red and 
white lines were associated for a time 
and then became dissociated. Even 
if a hyphal branch started with a sin- 
gle nucleus, mutation could occur dur- 
ing successive nuclear divisions and 
result in hetcrocaryosis. 

Whatever the origin of the hetero- 
caryotic condition, there is likely to be 
dissociation or regrouping of the differ- 
ent kinds of nuclei. An example: If 
the different kinds of nuclei in a hypha 
arc designated as A, B, and C, branches 
may be produced which contain any 
one of tlie three, or any combination 
of two, or all three of them. Conse- 
quently, lines with different characters 
can be isolated from such cultures. 
Hetcrocaryosis is to be distinguished 
from the dicaryotic condition in which 
two nuclei of opposite sex are paired 
and divide simultaneously during 
growth of the fungus, so that all of 
the derivatives of a given dicaiyotic 
cell also have paired nuclei of oppo- 
site sex that are kept close together by 
some attraction which seems to be 
lacking when sex is not involved as in 
hetcrocaryosis. 

Variation due to hybridization is 
common and extensive in plant patho- 
genic fungi. Many recombinations re- 
sult from cro.sscs between different bio- 
types, races, and varieties within some 
species, between s|:)ecics, and between 
some genera. Intraspccific hybridiza- 
tion has been studied especially in cer- 
tain rustr, smuts, and in Venturia spp. 
that cause scab of apples and pears. 
Interspecific and intergeneric crosses 
have been studied most extensively in 
the smuts. 

Hybrid i;:ation within species can re- 
sult in changes of pathogenicity be- 


cause of the production of numerous 
new biotypes and races, with a widen- 
ing of host range or increase of viru- 
lence for certain varieties of crop plants. 
It also can result in recombinations for 
many physiologic and morphologic 
characters. The practical importance 
of hybridization in nature is clearly 
apparent in certain smuts and rusts, 
and, together with mutation, may re- 
sult in. important changes in the reac- 
tion of varieties of crop plants to dis- 
ease. In the smuts, recombination of 
morphologic characters used in iden- 
tification of species can complicate 
problems of classification. 

In the rusts and smuts there are di- 
caryotic hybrids, as the two nuclei of 
opposite sex remain associated with- 
out fusing during the parasitic life of 
the fungus. In some fungi, including 
the smuts and rusts, distinct pha.ses in 
the life cycle arc associated with the 
nature and number of nuclei in the 
cells, as explained partly earlier. The 
terms used to designate these phases 
are haplophase, dicaryophase, and dip- 
lophase. 

In the haplophase of smuts and rusts, 
each cell has a single nucleus in the 
haploid condition — it has half as many 
chromosomes as the diploid nuclei and 
is therefore comparable with either a 
sperm nucleus or an egg nucleus. 
Prerequisite to sexual reproduction 
and to hybridization, therefore, are 
nuclei of at least two kinds sexually. 
In the smuts and rusts there are no 
visible differences loetween the sexes. 
The only known test of sexual differ- 
ence is performance, the pairing and 
fusion of the haploid nuclei. In both of 
these groups the sexual process extends 
over a long time because nuclei of 
opposite sex pair and remain paired a 
long time I)efore they complete the 
sexual act by fusing. 

The phase of development in which 
the cells and spores have paired hap- 
loid nuclei of opposite sex is the dicary- 
ophase, which means the two-nuclei 
phase. The mycelium in this condition 
is sometimes called a dicaryophyte. 
When the two haploid nuclei fuse, they 



PtOBLEMS OF VARIABILITY IN FUNGI 


produce diploid nuclei, containing dou- 
ble the number of chromosomes, half 
from one haploid nucleus and half 
from the other. The stage in which the 
cells have diploid nuclei is the diplo- 
phase. Thus, if black and pink haploid 
sporidial lines of smut of opposite sex 
are mixed together, the sporidia will 
conjugate in pairs and thus bring to- 
gether the nuclei B and P. The dicary- 
ophase then has paired nuclei, B+P, 
and is called a dicaryotic hybrid be- 
cause the hybridization is not com- 
plete until the B and P fuse to make 
BP. Similarly, if races i and 2 of the 
wheat stem rust are crossed, the result- 
ing dicaryotic hybrid has in each cell 
one nucleus of race i and one of race 2, 
and the pathogenicity is determined by 
this dicaryotic hybrid. The nuclei of 
races i and 2 do not fuse until telio- 
spores are formed. I'he diploid nu- 
cleus resulting from the fusion has fac- 
tors for pathogenicity from both race 
I and race 2; the hybridization is com- 
plete and segregation and recombina- 
tion of factors can occur, and new races 
can result. 

In the rusts the aecial and uredial 
stages are dicaryotic. As the parasitic 
uredial stage can propagate asexually, 
the characters of dicaryotic hybrids 
(dicaryophytes) can be studied. In the 
smuts, only the saprophytic haplophase 
can be propagated clonally, except for 
some diploid lines of Ustilago zeae. In 
both groups the parasitic dicaryophase 
terminates with the fusion of the two 
nuclei in the cells and the production 
of the diploid tcliospores, usually called 
chlamydospores in smuts, which pro- 
duce only a promycelium, or basidi- 
um, on which the gametic sporidia are 
formed following reduction division. 
In these fungi, then, the terms dicary- 
otic hybrids, hybrid tcliospores, and 
hybrid chlamydospores are often used. 

Studies of hybridization between 
biotypes within species arc most feas- 
ible in those smut fungi that produce 
haploid sporidia on a four-celled pro- 
mycelium, such as Ustilago z^m and 
Sphacelotheca sorghi. When two haploid 


57 

lines are crossed and the four primary 
sporidia from promyceiia of the result- 
ing diploid chlamydospores are re- 
moved and permitted to multiply on 
nutrient media, segregation for cul- 
tural characters is evident in the re- 
sulting colonies. When the haploid 
segregates arc mated, the results of 
segregation for sex factors are evident. 
All possible segregation ratios for cul- 
tural characters, for sex, and for muta- 
bility may occur. All four lines may be 
different, all may be alike, or there 
may be a 2:2, 3:1, or 1:3 segregation 
for each character, with all possible 
combinations between characters. All 
four lines may appear sexually identi- 
cal on the basis of intragroup matings, 
but they may prove different when 
mated with other tester lines, as there 
arc multiple sex groups in both species. 
As many as 40 to 60 haploid segregates 
from a single cross of Ustilago z^ae have 
been studied and all were different 
and did not include parental types. 
Although mutation may account for 
some of this diversity, there is con- 
clusive evidence that there can be very 
extensive recombinations of almost all 
characters studied, as shown by investi- 
gations made at the University of 
Minnesota by L. J. Tyler, Syed Vahee- 
duddin, M. A. Petty, and M. F. Kern- 
kamp, which will be summarized. 

Tyler made 10 crosses between 
monosporidial lines from the promy- 
ceiia of three chlamydospores of Spha- 
celotheca sorghi taken from a single 
smutted kernel. The progeny differed 
from each other in size of chlamydo- 
spores, size and hardness of sori 
(smutted kernels of sorghum), time 
required for spore germination, and 
degree of pathogenicity. Vaheeduddin 
produced gray, brown, and gray- 
brown peridia (the membrane sur- 
rounding smutted kernels) by inocu- 
lating sorghum with different combi- 
nations of these ifionosporidial lines 
and also produced a parasitic race 
clearly different from anything previ- 
ously deifcribed. Had this race been 
produced in nature, it would have in- 
creased the pathogenicity of the 



VEARBOOK OF AOIICULTURE I9SE 


58 

“chJamydospore race,” and appro- 
priate differential hosts would have 
been needed to detect and separate it. 

Petty and Kemkamp proved that 
promycelial characters of U. zeae var- 
ied with the combinations of mono- 
sporidial lines that produced them, 
and other investigators have shown 
that pathogenicity, chlamydosporc 
production, size and color of sporidia, 
and tendency to produce sporidia or 
mycelium vary widely with the cross. 
Kernkamp, as an example, crossed 
sporidial and mycelial lines of U, zeae^ 
and sporidia and hyphal branches were 
formed in all possible ratios on the 
promycelia of the hybrid chlamydo- 
spores. 

Despite the wide diversity of haploid 
lines in smuts, group characters may 
be fairly constant. In experiments 
made at the University of Minnesota, 
chlamydosporc collections of U. zeae 
from Ohio, Kansas, Minnesota, and 
other States retained their distinctive 
pathogenicity when several varieiies 
and lines of corn were inoculated with 
three successive annual crops of chlam- 
ydospores. Likewise, collections from 
Minnesota, Wisconsin, Louisiana, and 
Mexico tended to produce haploid 
lines with group cultural characters: 
Most lines from Wisconsin were light 
in color; those from Louisiana were 
predominantly dark; those from Min- 
nesota were intermediate. There ap- 
pear, therefore, to be group characters, 
with great diversity within the group; 
and the group may change with vary- 
ing degrees of readiness. The new race 
of Spkacelotheca sorghi synthesized by 
Vaheeduddin, for example, easily 
could have changed the group char- 
acter and could have been isolated 
from the group only by the selective 
effect of differential hosts. 

At the University of Wisconsin, 
G. W. Keitt and others made crosses 
between single ascospore cultures of 
the apple scab fungus, Venturia inaequa- 
Us. From the resulting perithecia the 
parental types and new types w^erc 
isolated. Factors for pathogenicity seg- 
regated in the first or second nuclear 


division in the ascus, and factors for 
the infection type produced by each 
segregate on a given host appeared to 
be in a single locus, with multiple 
alleles determining infection types on 
different varieties of apples. Langford 
and Keitt made crosses similarly be- 
tween single-ascospore lines of V, 
pyrina^ pear scab, and segregates dif- 
fered in pathogenicity for certain vari- 
eties of pears, in time required for 
formation of the perithecia, and in 
numbers of ascospores produced. 

S. P. Chilton and others at the 
University of Louisiana also obtained 
evidence for hybridization between 
different isolates of Glomerella. 

In 1928 V. Goldschmidt in Germany 
crossed two races of Ustilago violacea 
smut of pinks, one of which attacked 
Silene saxifraga but not Melandrium alba; 
and the other attacked Melandrium but 
not Silene. The hybrid, however, at- 
attacked both hosts. Similarly, C. S. 
Holton and H. A. Rodenhiser, of the 
Department of Agriculture, crossed 
race T8 with Tg and race T8 with 1 10 
of Tillelia caries, the reticulate-spore 
stinking smut of wheat. Some of the 
hybrids attacked the wheat cross 
Hus.sar x Hohenheimer, hitherto re- 
sistant to all known races of lx)th 
T. caries and T. Joetida, the smooth- 
spore species of stinking smut. 

Races of wheat stem rust can cross 
readily and thus result in the produc- 
tion of many biotypes or races, includ- 
ing some that are new. Races of rust 
may or may not be dicaryotic hybrids. 
Some are homozygous and others are 
very heterozygous. Thus, races 9 and 
36 arc relatively homozygous and usu- 
ally breed true when they are selfed; 
but race 53, which is highly heterozy- 
gous, has segregated into as many as 18 
races when “selfed” in the sexual stage 
on barberry. Many experiments have 
been made to determine the results of 
cros.sing races of wheat stem rust, and 
in 1929 W. L. Waterhouse concluded 
from experiments made at Sydney 
University that new races could be 
produced by crossing known races. 
Similarly, Margaret Newton and T. 



PtOBlEMS OF VAtlABILITY IN FUNOI 


Johnson made extensive experiments 
at the Dominion Laboratory of Plant 
Pathology to determine the number 
and kinds of races produced by crossing 
known races and to discover the ge- 
netic principles involved. When the 
relatively homozygous races 9 and 36 
were crossed, the first generation hy- 
brid was race 17, but when this vyas 
selfed it produced race 36, race 1 7, and 
six other races. Newton and Johnson 
studied the dominance of factors for 
virulence by crossing races, one of 
which could attack certain varieties 
that the other could not. Thus, the 
nonvirulcncc of race 9 for Kanrcd 
wheat was dominant over the virulence 
of race 36, but the virulence of race 9 
for three varieties of durum was domi- 
nant over the nonvirulence of race 36, 
and the nonvirulence of race 36 for 
Vernal emmer was dominant over the 
virulence of race 9. If factors for viru- 
lence in one of the nuclei of the di- 
caryophase are recessive and domi- 
nated by those for nonvirulcncc, a race 
may produce races more virulent than 
itself after being “selfed” in the sexual 
stage on the barberry, because the 
more virulent races could be “double 
recessives.” 

That hybridization and segregation 
in Puccinia graminis arc going on in 
nature is clear from studies of races and 
biotypes on and near barberry bushes. 
E. C. Stakman and W. Q. Loegering, 
in extensive studies made cooperatively 
by the Department of Agriculture and 
the University of Minnesota, identified 
43 races and bio types of P. graminis 
triiiciy wheat stem rust, in 1949, in the 
immediate vicinity of three groups of 
barberry bushes in Lebanon County, 
Pa., and only five races in nonbarberry 
areas of the State. In 1940 they iso- 
lated races 9, 10, 14, 24, 40, 55, 69, 77, 
79» B3, 1 1 7, 125, 126, 140, 146, and 
147 from or near barberry bushes but 
not elsewhere in the United States and 
Mexico. The ratio between races and 
uredial collections is about i :5o; that 
for aecial material is about 1 15 or less. 
Although only 14 races of P. graminis 
avenae arc known, the presently de- 


59 

structive races 7, 8, 10, and 12 were 
found repeatedly near barberry before 
they became generally distributed; the 
potentially dangerous races 6 and 13 
were still found only on or near bar- 
berry bushes during the growing season 
of 1952, but they are likely to become 
more widely distributed in future. 

That there are extensive recombi- 
nations in the sexual stage of autoe- 
cious rusts, those that produce all 
stages on the same plants, is shown by 
the researches of H. H. Flor on flax 
rust, Melampsora lini^ made at the 
North Dakota Agricultural Experi- 
ment Station. Flor isolated 64 races 
from the F2 progeny of a cross be- 
tween race 22 from South America 
and race 24 from the United States. 
Of these, 62 were previously unknown, 
and some were more virulent on cer- 
tain varieties of flax than either 
parental race. 

Many crosses have been made be- 
tween varieties of Puccinia graminis^ 
principally between the varieties tritici 
and secalis. Although there is consider- 
able information on parasitic behavior 
of the immediate crosses, little is 
known about their subsequent prog- 
enies. Many of the varietal hybrids 
possessed the pathogenic capabilities 
of one or the other parental variety, 
while others were less pathogenic, be- 
ing more or lc.ss intermediate between 
the two parents; but a few differed 
strikingly in their parasitic capabili- 
ties. In some crosses, many new races 
were produced in the Fi generation, 
which is expected if one or both of 
the parents are heterozygous. 

At the University of Minnesota nine 
physiologic race.s were obtained by 
crossing the homozygous race 36 of 
the tritici variety with agrostidis (red- 
top) variety. All of the nine races 
infected wheat, but none infected red- 
top or other grasses of the genus Agros- 
tis. From a cross between race 36 of 
wheat stem rust (tritici) and race ii 
of rye stem rust (secalis), eight races of 
tritici and two of secalis were isolated. 
Two of the tritici races were new and 



YEARBOOK OP AORICULTUKE 1953 


6o 

intermediate in pathogenicity between 
the parents. Similar results have been 
obtained at the Dominion Laboratory 
of Plant Pathology at VVinnipeg, Can- 
ada. The varieties tn/tet and sfca/is are 
highly interfertilc and seem closely re- 
lated, as both can infect barley and 
several grasses about equally well. One 
hybrid between the two attacked 
barley, but not wheat or rye, and 
could therefore be designated as vari- 
ety hordei. From another cross between 
the tritid and secalis varieties, M. N. 
Levine and R. LT. Cotter isolated a 
hybrid race that attacked wheat, rye, 
and barley heavily, thus combining 
the pathogenicity of both parents. 

T. Johnson and Margaret Newton 
in Canada crossed tritid and avenac 
(oats variety) and obtained a hybrid 
that attacked certain varieties of both 
wheat and oats, whicli neither of the 
parents could do. Although the host 
range of the hybrid was wider than 
that of either parent, pathogenicity 
was weaker than that of the parents 
on their respective hosts. More studies 
on varietal crosses are needed. 

At least eight species and genera 
of cereal and grsss smuts have been 
crossed : 

Ustilago avenae x U. kolleri (loose smut 
X covered smut of oats). 

Ustilago avenat x U. perennans (loose 
smut of oats x smut of tall oatgra.ss). 

Ustilago hordd \ U. nigra (covered 
smut X false loose smut of barley). 

Tilleiia caries x T. Joetida (low bunt 
X high bunt of wheat). 

Sphacelotheca sorghi x S, cruenta (cov- 
ered kernel smut x loo.se kernel smut 
of sorglium). 

Sphacelotheca sorghi x Snrosporium r<r- 
iliamtm (covered kernel smut x head 
smut of sorghum). 

Sphacelotheca cruenta x Sorosporiwn 
reilianum (loose kernel smut x head 
smut of sorghum). 

Sphacelothica destruens x S. syntherismae 
(two head smuts of grasses). 

Spccie.s involved in interspecific 
cros.ses differ considerably in morpho- 
logical and physiological characters; 


therefore they arc suitable for study- 
ing the recombination and segrega- 
tion of two or more distinct characters, 
such as type and consistency of the 
sori (spore masses), markings and color 
of chlamydospores, and pathogenicity. 

Fertile interspecific dicaryotic hy- 
brids are easily made between the 
eight combinations listed above. In 
some instances the Fi chlamydospores 
germinate normally; in some cases 
chlamydospores and sporidia germi- 
nate poorly or abnormally. 

The inheritance of factors for 
spore- wall markings of hybrid chlamy- 
dospores of smuts usually is simple. 
When one with smooth-walled spores 
is crossed with one that has spiny- 
walled spores (cchinulaie), factors for 
spiny wain tend to dominate over those 
for smooth wall, but in crosses between 
species of Tilletia with smooth and 
reticulate-walled spores (those with 
netlike ridges) more than one set of 
factors apparently is involved. In 
some cros.ses smooth completely dom- 
inates over reticulate, and in others 
reticulate is partly domiikant over 
smooth, and intergrading types of re- 
ticulation may occur in the Fi and ¥% 
chlamydospore generations. 

I'he F 1 sori (spore masses) produced 
by interspecific crosses cf smuts tend to 
be intermediate between the two par- 
ents. There may, however, be consid- 
erable variation, depending on the 
species involved in the cross. Different 
dicaryophytes of the same cross also 
may produce sori of several shapes and 
sizes. In the F2 generation, and some- 
times in later ones, there frequently are 
many diverse types. The factors for 
different sorus characters are usually 
inherited independently of each other 
and of tho.se for sex and pathogenicity; 
hence, new combinations of morpho- 
logical and pathogenic characters are 
common. 

The range of variation in types of 
smut is well illustrated in crosses tie- 
tween the loose and covered smuts of 
oats. When varieties of oats are inocu- 
lated with different combinations of 



FROilEMS OF VARIABILITY IN FUNGI 


monosporidial lines of Ustilago avenae^ 
loose smut, and U, kolleri, covered 
smut, many kinds of smutted heads 
(panicles) may be produced, ranging 
from loose to covered, through many 
intermediate types. One combination 
may produce loose smut on one variety 
and covered smut on another, and 
another combination may produce the 
same type of smut on both varieties, 
which indicates that the type of smutted 
panicles is determined by both the 
fungus and the variety of oats. 

In interspecific crosses between the 
barley smuts Ustilago hordei (covered 
type) and U, nigra (false loose type), 
made by C. C. Allison at Minnesota, 
the Fi head types were intermediate, 
but tended toward the loose type. The 
progeny from the Fi chlamydospores 
produced not only the parental types 
of sori, compact heads with smooth 
spores and loose heads with cchinulatc 
spores, but several new types, including 
intermed iates with smooth or echinulate 
spores, compact heads with echinulate 
spores, and loose heads with smooth 
spores, 

L. J. Tyler and C. P. Shumway, at 
Minnesota, made crosses between the 
sorghum smuts Sphaceloikeca sorghi 
(covered kernel smut) and Sorosporium 
reilianum (fxad smut). The characters 
of the F, sori and of the chlamydo- 
spores tended to be intermediate 
between those of the parents. Syed 
V^ahccduddin crossed Sorosporium reili- 
anum (head smut of sorghum) walh 
Sphaceloikeca cruenta (loose kernel smut 
of sorghum). Dificrent combinations 
of monosporidial lines produced sori 
differing in shape and size. Some of 
the Fi sori resembled one or the other 
parent, w'hilc others were intermediate. 
Similar types were obtained again in 
F 2 from inoculating with different 
combinations of fj (gametic) segre- 
gates. Naturally, there was segre- 
gation of factors for many other 
characters also, such as size and 
echinulation of chlamydospores, cul- 
tural characters, sex, and patho- 
genicity. Some of the hybrids between 
the two smuts had pronounced hybrid 


61 

vigor. They caused extreme elonga- 
tion of the ovaries of sorghum, thus 
producing an effect similar to that 
caused by the long smut of sorghum, 
Tolyposporium filijerum. Moreover, the 
chlamydospores, although intermedi- 
ate in size betw^een the sizes of the 
parental spores, germinated over an 
exceptionally wide range of tempera- 
ture and produced promycelia two and 
a half to three times as long as those 
of either parent, and the sporidia and 
hyphal branches were correspondingly 
large. In addition, some haploid 
segregates had extraordinary toler- 
ance for certain chemicals. Since 
sporidia and promycelial cells are 
haploid, not diploid as in higher 
plants, a study of hybrid vigor in smut 
fungi may aid in interpreting that in 
higher plants. 

Interspecific hybridization appar- 
ently produces new parasitic races of 
some smuts in nature. New virulent 
races of buff smuts of oats have been 
produced artificially by crossing Usti- 
lago avenae with U, kolleri. Some of the 
hybrids had a wider host range than 
either parent. Some of the hybrid races 
of the buff smut attacked both the 
susceptible variety Monarch, and the 
variety Gothland, which was immune 
to the parental buff race. The inter- 
specific hybrids combined some of the 
factors for pathogenicity of both 
parents. It appears that new races of 
buff smut can be produced readily 
by crossing any buff race with normal 
black races of U, avenae or of U. kolleri. 

Many new parasitic races of Tilletia 
that cause bunt or stinking smut of 
wheat have been produced by hybridi- 
zation between 7*. caries and T. 
Joetida. Although some hybrid races 
are less virulent than: either parent, 
others combine factors for pathogenicity 
of both parents. Crosses between T, 
Joetida and T. caries can result in new 
morphologic types also. As an ex- 
ample, the hybrid sori and chlamydo- 
spores were smaller in one cross chan 
those of either parent. Hybrids also 
may have varying degrees of speire- 



YEARBOOK OF AGRICULTURE 1953 


62 

wall reticulations, and size of chlamy- 
dospores can vary considerably. Su<^ 
hybrids occur in nature also, and in 
some cases have been given taxonomic 
rank, such as T. caries intermedia. 

That there are innumerable species, 
parasitic races, and biotypes of plant 
disease fungi is a banal truism. That 
innumerable new biotypcs and races 
can be produced by mutation, hybrid- 
ization, and heterocaryosis has been 
shown by extensive experimentation. 
That new biotypes and parasitic races 
often appear in nature is known from 
long-continued observ’ation. The new 
biotypes and races, even though they 
were produced infrequently, could still 
become widely prevalent in a short 
time because many fungi are prodi- 
giously prolific and can be dissemi- 
nated widely and quickly by the wind. 
A single kernel of smutted wheat 
contains between 2 million and 12 
million spores; a single pustule of 
wheat stem rust may contain a quarter 
of a million spores; stem rust may 
produce 70 billion spores on a single 
barberry bush; there are about 50,000 
billion rust spores on one acre of fairly 
heavily rusted wheat. Countless bil- 
lions of spores are literally carried on 
the wings of the wind. Many of the 
new biotypes that arc produced in 
these enormous populations are not 
dangerous, but some are. The poten- 
tially rapid multiplication of new 
biotypes and parasitic races of the 
fungi and the potentially rapid spread 
make the more virulent ones poten- 
tially dangerous, and too often poten- 
tiality has become reality. 

The practical implications of the vast 
variation in plant pathogenic fungi are 
manifold, but the most important is 
the menace to food supplies. Some of 
the most devastating epidemic disca.ses 
of basic food and feed crops can be 
controlled economically only by the 
development of disease-resistant vari- 
eties. New parasitic races have re- 
peatedly appeared to attack these 
varieties in the past and the menace 
still exists for the future. The genetic 
diversity ard phenotypic variability of 


many of the most destructive path- 
ogens are so great as to create extreme- 
ly complex problems of disease control 
in the present and to raise the question 
as to how complex they can become in 
the future. How much virulence can 
nature put into plant pathogens and 
how much resistance can man put into 
crop plants? This is one of the most 
important questions for present and 
future agriculture. It can be answered 
only by basic studies to determine the 
limits of genic combinations for viru- 
lence in pathogens and for productiv- 
ity and disease resistance in crop plants. 

£. C. Stakman is head oj the department 
of plant pathology and botany, University 
of Minnesota, and an agent in the United 
States Department of Agriculture, He has 
devoted his professional life principally to 
basic and practical studies of diseases of 
crop plants and the fungi that cause them. 
He is widely known for investigations on 
the epiderniology and physiologic specializa* 
lion in stem rust of wheat and the genetics of 
the smut fungi. At Minnesota and elsewhere 
he has participated in breeding programs for 
disease-resistant varieties, particularly of 
wheat and oats. He was president in ig^g of 
the American Association for the Advance- 
ment of Science, His current activities in- 
clude membership in the executive committee 
of the National Science Board, in the 
Advisory Committee for Biology and Medi- 
cine of the United States Atomic Energy 
Commission, and chairmanship of the Com- 
mittee on International Cooperation of the 
American Phytopathological Society. 

J. J. Christensen is professor of plant 
pathology at the University of Minnesota, 
where he obtained the doctor of philosophy 
degree in rg2^. He has made extensive 
studies of cereal diseases and the genetics of 
plant pathogens in relation to breeding for 
resistance and has participated in cereal 
breeding programs. In addition to work at 
Minnesota, Dr. Christensen has studied 
genetics of plant pathogens in Europe, has 
been adviser to SCAP on plant diseases in 
Japan, and has traveled extensively in 
South America on a study of cereal diseases. 
He is past president of the American 
Phytopathological Society, 



lAtTEtIA, FUNGI. AND INSECTS 


Bacteria, 
Fungi, and 
Insects 


J. G. Leach 

Insects influence fungus and bac- 
terial diseases of plants in several ways. 

Without disseminating the micro- 
organisms themselves, insects may 
make wounds on a plant through 
which fungi or bacteria may enter. An 
example is white grubs, which often 
feed on the roots of raspberry plants 
and make wounds through which the 
bacteria that cause crown gall gain 
entrance from the soil. We have no 
evidence, however, that the white 
grubs arc involved in the spread of 
the bacteria. Curculios that feed on 
young peaches and plums likewise 
make wounds through which wind- 
borne s|X>res of the brown rot fungus 
enter and cause infection. 

But some insects spread pathogens 
from diseased plants to healthy plants 
without wounding the plants. When 
bees transmit the bacteria causing fire 
blight of apples and pears from dis- 
eased to healthy blossoms, they make 
no wounds on the blossoms but intro- 
duce the bacteria into the nectar, 
where they grow and later penetrate 
the tissues through the nectar glands. 
Another example is the relationship 
between flies and the ergot disease of 
rye. In early stages of infection, the 
fungus produces large quantities of 
spores in a sugary exudate, which 
has a foul and catrionlike odor and 
attracts flics. The flics feed on it and 
become contaminated with the fungus 
spores. They transport the spores to 
healthy flowers, where infection takes 


63 

place without the need of any wounds* 

More efficient is the insect that 
transmits the pathogen from plant to 
plant and also makes the wound 
through which infection takes place. 
That kind of relationship exists be- 
tween the elm bark beetles and the 
Dutch elm disease, the striped cucum- 
ber beetle and the bacterial wilt of 
cucurbits, and many other diseases 
transmitted by insects that feed upon 
the plants by chewing the tissues or 
sucking the sap. 

In a few instances insects may in- 
fluence the development of a disease 
although they neither disseminate the 
pathogen nor make wounds through 
which it enters the plant. So it is 
when beetles of Monochamus species 
bore into the heartwood of trees or 
logs already infected with wood-rot- 
ting fungi and hasten the growth of 
the fungi and consequently the rotting 
of the log. 

Almost everywhere, except possibly 
the humid Tropics, some period of the 
year is unfavorable for the growth of 
fungi and bacteria. Then the pathogen 
has the problem of survival. In the 
Nortli the critical period is winter, 
when temperatures arc too low for 
growth of both pathogen and host. In 
the warmer, drier regions, heat and 
drought may be limiting factors. The 
successful pathogens are the ones that 
have some special adaptations that 
enable them to survive winter cold 
and summer heat so that they are 
ready to cause infection when condi- 
tions again arc favorable. Some patho- 
gens are adapted to survival in the 
soil. Others survive in roots or stems 
of perennial plants or in the seeds of 
annual plants. Some produce resistant 
spores that withstand unfavorable 
conditions. 

Some fungus and bacterial patho- 
gens that are transmitted primarily by 
insects can survive within the body of 
the insects that transmit them. The 
bacteria that cause wilt of sweet corn 
survive the winter in the bodies of the 
corn flea beetle. Those that cause the 
wilt of cucurbits survive in the bodies 



YEAtftOOK OF AGIICUITURE 1953 


64 

of the striped and spotted cucumber 
beetles. The fungus that causes the 
Dutch elm disease may overwinter in 
the body of the elm bark beetle. Fungi 
that cause the blue stain disease of 
pines survive in the bodies of the pine 
bark beetles. Pathogens like these, 
which survive within the bodies of 
their insect vectors, withstand the 
digestive fluids in the insects, while 
many other micro-organisms arc killed 
and digested. 

When fungi and bacteria survive in 
the bodies of their insect carriers, there 
is often a mutualistic symbiotic rela- 
tionship — a mutual aid arrangement. 
The fungi or bacteria may supply 
digestive enzymes or vitamins that the 
insect needs. They may condition trees 
so that insects may breed in them or 
they may provide a more concentrated 
source of nitrogen for the insects. In 
return, the insect protects the micro- 
organism against unfavorable environ- 
ments, transmits them to susceptible 
plants, and often makes the wounds 
through which they infect. 

Such associations between insects 
and plant pathogens arc not matters of 
chance. They are the result of a long- 
evolutionary process. Sometimes in- 
sects have developed special organs in 
their bodies for the purpose of harbor- 
ing the micro-organism in relatively 
pure culture. The females of some 
species that have become dependent 
upon micro-organisms have special 
organs and devices in their bodies 
designed to contaminate the eggs so 
that the new generation will have a 
supply of the necessary micro-organ- 
isms. A rot of apples associated with the 
apple maggot fly is caused by bacteria 
that are transmitted in this way. The 
female fly has special pouches in the 
walls of the oviduct that harbor the 
bacteria and are so arranged that they 
provide a mechanism that contami- 
nates each egg with the bacteria as it is 
deposited into the tissues of the apple. 

Wind, water, man, and animals also 
spread plant diseases. Only a few 
fungus and bacterial diseases depend 
entirely on insects for their spread and 


development. Wind scatters spores of 
pathogenic fungi, but only a small per- 
centage of them fall upon the proper 
plant under conditions necessary for 
infection. Most of them are wasted. 
But spores that are adapted to dissem- 
ination by insects are taken usually 
directly to a plant that is susceptible to 
their attack and are often deposited in 
wounds where infection occurs imme- 
diately. Thus, wind dissemination is 
much more wasteful of inoculum and 
is more subject to the vagaries of the 
weather; insect dissemination leaves 
less to chance and is more economical 
of inoculum. The situation is like the 
pollination of flowering plants by wind 
and by insects. The highly developed 
adaptative mechanisms of insect pol- 
lination have evolved from the more 
primitive and less efficient mecha- 
nisms of wind pollination. A similar 
evolutionary trend appears to exist in 
the methods of dissemination of fungus 
and bacterial di.seases. 

A more striking parallel with insect 
pollination is found in the rust fungi, 
in which insects are instrumental in 
transporting the sexual elements of the 
pathogen in a manner quite compa- 
rable to insect pollination of flowering 
plants. Many of the rust fungi are het- 
erothallic, which means that they form 
two kinds of mycelium that differ sex- 
ually. Before the rust fungus can com- 
plete its entire life cycle, pycniosporcs 
of one sex must be transported to re- 
ceptive hyphac of the opposite sex so 
that fertilization can be accomplished. 

The phenomenon has been studied 
in the black stem rust of cereals (Pmc- 
cinia graminis). The rust is hetcroecious, 
which means that it lives part eff its 
life on cereals and grasses and part on 
the common barberry (Berberis vul^ 
garis). The spores that infect the bar- 
berry leaf arc of two sexes, designated 
as and — . They arc in the haploid 
condition and neither one can com- 
plete the life cycle until there has been 
a sexual fusion of the two. The + or — 
spore infects the barberry leaf and 
forms a mycelium of its own kind ( + 
or — ). From them are formed (on the 



BACTERIA, FUNGI. AND INSECTS 


upper side of the barberry leaf) small 
fniiting bodies (pycnia), each of which 
produces myriads of pycniospores of 
the corresponding sex. Each pycnium 
produces also a number of short hy- 
phae, which protrude from the pyc- 
nium and serve as receptive organs com- 
parable to the stigma of flowering 

lants. They are known as receptive 

yphae. Before aeciospores can be pro- 
duced on the under side of the leaf, a 
pycniospore of one sex must reach a 
receptive hyphae of the opposite sex 
and fuse with it. The nucleus from the 
pynciospore passes into the receptive 
hypha and fuses with a nucleus of the 
opposite sex. In this process it fertilizes 
or “diploidizes” the mycelium within 
the leaf tissues so that aeciospores can 
be formed. 

Since the pycnia producing these sex- 
ual organs of different sexes are often 
separated on a leaf or occur on differ- 
ent leaves, a way is needed to bring 
the pycniospores to the right kind of 
receptive hypha. Nature has provided 
a mechanism that tends to insure suc- 
cessful fertilization. The pycnia arc 
produced on the upper leaf surface on 
a bright orange spot. The pycniospores 
are liberated in a drop of nectar that 
has a high sugar content and a fragrant 
odor. The bright spots, the fragrance, 
and the food attract insects of many 
kinds, especially flies. The insects in 
feeding move from one pycnium to an- 
other and by transporting the spores 
to receptive hyphae bring the opposite 
sexes together. 

Since individual diseases may be 
transmitted in several ways, it is desir- 
able to know the relative importance 
of the different methods. Such infor- 
mation often is necessary for working 
out effective control measures. It has 
been proved that the bacteria causing 
wilt of cucurbits survive the winter 
only within the bodies of cucumber 
beetles and in nature are transmitted 
only by the beetles. It is obvious there- 
fore that eflective control of the beetles 
will control the disease. But when in- 
sect transmission is one of .several 
methods, the relative importance of 


65 

each must be established and control 
measures modified accordingly. An ex- 
ample: The seed-corn maggot is known 
to transmit blackleg of potatoes and 
other bacterial soft rots, but it is only 
one of several means of transmission 
and even though the insects were com- 
pletely controlled the diseases would 
be spread to some extent by other 
means. 

It is especially difficult to determine 
accurately the importance of an insect 
vector in relation to other means of 
spread if one cannot control the vec- 
tor. We knew for a long time, for 
example, that the brown rot of peaches 
might have some connection with the 
feeding and egg-laying of the curculio, 
but since no effective means of con- 
trolling the insect was available we 
could get no accurate data as to its 
importance. When the newer organic 
insecticides like DDT became avail- 
able, however, better control of the 
curculio was possible, and we learned 
that less brown rot occurred in or- 
chards in which the curculio was 
checked. 

We have no evidence that the cur- 
culio is of great importance in dis- 
seminating the spores of the brown rot 
fungus, which are readily wind-borne, 
but it is obvious that the curculio 
influences the development of brown 
rot by making wounds on immature 
fruits through which the wind-blowm 
spores are able to infect. The brown 
rot fungus has difficulty in infecting 
immature fruits if the skin is unin- 
jured, but it grows readily in punc- 
tures made by the curculio. Spores 
formed early on the injured green 
fruits provide an abundant source of 
inoculum for the ripening fruit later 
in the season 

Often more than one. kind of insect 
may transmit the same disease. Since 
1891, when M. B. Waite, a pioneer in 
the Department of Agriculture, first 
showed that the honey bee could trans- 
mit fire blight of orchard fruits, much 
has been written, pro and con, about 
the role of bees in the spread of the 
disease. The importance of the bee is 



YIARIOOK OP AORICUlTUm 19S3 


66 


often overemphasized by failure to 
recognize that other insects regularly 
transmit the disease and that wind, 
rain, and other agents also are in- 
volved. The bacteria commonly live 
over winter in cankers on the larger 
branches of the tree from which they 
are liberated in early spring in a sticky 
exudate. 

Flies and ants that feed on the 
exudate are chiefly responsible for 
transporting bacteria from the cankers 
to the blossom and for initiating the 
first blight infection of the new growth. 
Bees and wasps, flics, and other in- 
sects that visit the blossoms transmit 
the disease from blossom to blossom. 
Aphids, leafhoppers, and other suck- 
ing insects transmit it to the young 
green shoots. In the meantime, wind- 
blown rain may also spread the bac- 
teria throughout the tree and man in 
his pruning operations may aid in 
spreading it throughout the orchard. 
Transmission of this disease is a com- 
plicated process, and proper evalua- 
tion of the different means of spread, 
although difficult, is essential to an 
understanding of its nature. 

Those who have studied the Dutch 
elm disease have shown that several 
different insects under some condi- 
tions may traasmit the disease. In the 
United States the principal vector is 
the smaller European elm bark beetle 
(Scofylus multi striatus). The native elm 
bark beetle {Hylurgnpinus rufipes) may 
also transmit the disease, but is much 
less effective. The greater efficiency 
of the former is due to its feeding 
habits. 

Both insects breed under the bark of 
infected trees and the fungus pathogen 
p-ows and forms spores in their breed- 
ing tunnels so that the new broods of 
each species have equal opportunities 
for becoming contaminated with the 
fungus spores. But when the new crops 
of beetles emerge, their behavior is 
quite different. Those of the European 
species fly to healthy trees and feed on 
the young branches and in the process 
inoculate the tree with the disease. 
Those of the native species, however, 


do not feed on young twigs. They may 
bore into the trunk of healthy trees 
but they rarely penetrate deep enough 
to reach the sapwood and inoculate 
the tree. Neither insect is able to es- 
tablish breeding tunnels in healthy 
trees, but those trees which were in- 
oculated by the Europiean species 
when it fed on the small t>ranches are 
weakened so that they are subject to 
the attack of both species. These 
diseased and beetle-infested trees sup- 
ply a new brood of contaminate 
beedes. Thus a slight difference in 
feeding habit may gready influence 
the efflciency of an insect vector. 

In a study of insect transmission of 
plant diseases, consideration must 
therefore be given to the feeding and 
breeding habits of all the insects that' 
arc potential vectors. Many other 
insects develop in infected trees and 
have abundant opportunities to be- 
come contaminated with the pathogens 
but are of no significance as vectors. 
For example, the wood-boring beetles 
(Buprestidacand Cerambycidae) breed 
in infected trees and come in contact 
with the pathogenic fungus, but when 
they attack the new tree the adult 
female of the wood -boring beetle does 
not bore into the tree but deposits her 
eggs on the surface or in niches in the 
bark. When the eggs hatch the young 
larvae bore into the tree, and because 
they have had little or no opportunity 
to become contaminated with the 
fungus they rarely serve as vectors. 

More than 40 species of insects visit 
rye blos.soms and feed on the spores of 
the ergot fungus, but not all have equal 
importance as carriers of ergot. Among 
those that feed on the spores arc certain 
species of pollen-eating flies. I'hey also 
regularly visit healthy flowers and feed 
on the pollen. In doing so they trans- 
mit the disease. Thus they are more 
effective vectors than those that do not 
eat pollen and visit healthy flowers only 
by chance or not at all. 

An important aspect of plant pathol- 
ogy is the study of the influence of 
environment on disease. When the 
diseases are caused by fungi or bacteria 



BACTCRIA. FUNGI, AND INSECTS 


the study is complicated by the efiectof 
the environment on two different 
organisms, the micro-organism and the 
crop plant, and also on the interaction 
of the two. If the disease is transmitted 
by an insect, one has to study three 
different organisms and their inter- 
actions. 

The prevalence of a plant disease can 
be determined more by the influence of 
the environment on the insect vector 
than by its influence on the disease 
itself. That is obvious in such diseases 
as bacterial wilt of cucurbits, for which 
insects are the only known means of 
transmission. Any weather condition 
that influences the abundance of the 
cucumber beetles must also influence 
the prevalence of the disease. 

Bacterial wilt of sweet corn is more 
prevalent following mild winters than 
it is following cold winters. It is gener- 
ally agreed that this is because of the 
survival in mild vvintei*s of a greater 
number of contaminated flea beetles. 

Unexpected siiuatiotis often happen 
when environment modifies strikingly 
the activities of insect vectors. The soft 
rot of vegetables, caused by bacteria, 
is usually favored by wet weather, but 
when the disease occurs as a heart rot 
of celery it is most destructive in dry 
weather. Is was hard to understand 
why until it was discovered that, on 
celcr>% the disease is tran.smittcd by 
insects of the fruit fly group, I'he flies 
deposit their eggs on the celery leaves. 
When the eggs hatch, the young mag- 
gots burrow into the leaves and inocu- 
late the plant with the bacteria. The 
insects will deposit their eggs only on 
moist leaves. In wet weather the eggs 
are deposited on the outer leaves of 
the celery plant where the decay cau.scs 
little damage to the plant because the 
outer leaves are resistant to decay and 
arc removed and discarded at harvest. 
In dry weather, how■e^'er, when the 
outer leaves are not wet, the in.secis 
seek the moist heart leaves on w'hich to 
lay their eggs. Wh^n the lar\-ae inocu- 
late the heart leaves with the bacteria 
the growing point is killed and stem 
elongates, so that the plant has no 


67 

market value. Thus a disease that 
normally is favored by moist weather is, 
in this case, more destructive in dry 
weather. 

Insects also may indirectly influence 
the spread of diseases by birds. Chest- 
nut blight is spread over short dis- 
tances by wind and water, but spot 
infections have appeared x oa miles or 
more from known infected areas. They 
were started by the woodpeckers and 
sapsuckers whose beaks had been con- 
taminated w'ith spores while feeding 
on insects that were breeding in can- 
kers on infected trees. Since the birds 
feed on both insects and healthy cam- 
bium, they are effective vectors. After 
feeding on insects in a canker and con- 
taminating their beaks with spores, 
they would continue their migrations 
and fly many miles before pecking 
holes in the bark of healthy chestnut 
trees to feed on the cambium. But in 
so doing they would effectively inocu- 
late the trees with fhe spores adhering 
to their beaks. 

It is evident from this brief review 
that insects play an important role in 
the spread and development of plant 
diseases. Successful control of many 
plant disca.ses depends upon the con- 
trol of the insects that spread them or 
otherwise influence their development. 
Sometimes the in.secl that transmits a 
plant disease is not directly injurious 
and would be of no economic impor- 
tance if it did not transmit the disease. 
Frequently there is a symbiotic rela- 
tionship between the insect and the 
disea.se that it transmits in w'bich both 
the insect and the micro-organism 
derive mutual benefits. 

J. G. Leach has been head of the 
department of plant pathology and hac^ 
terioiogy in West Virginm University since 
Earlier he was professor of plant 
pathology in the University of Minnesota. 
He has done extensive research with in^ 
suts in relation to plant diseases. In ig^o 
he published a textbook^ Insect Trans- 
mission of Plant Diseases. He is a 
former president of the American Phyto* 
pathological Society. 



68 


YfAtBOOK OF AOtICUlTUit 19$S 


Crown Gall — 
a Malignant 
Growth 


A. J. Riker^ A. C. Hildebrandt 

Crown galls arc diseased growths 
that occur on peaches, apples, rasp- 
berries, roses, sugar beets, and a great 
many other broadleaved plants. The 
galls appear commonly where the 
plants come out of the ground, the 
crowm — hence the name. 

They ordinarily are quite soft. They 
have neither a definite exterior like a 
bark layer nor a woody interior like a 
stem. Having no protection against sec- 
ondary invaders, the galls become hosts 
to various bacteria, fungi, and even in- 
sects, particularly during w'et weather. 
A type of decay like soft rot sets in. 

A gall contains a disorganized mix- 
ture of large, sw'ollen cells; small cells 
that divide rapidly; and sap-conduct- 
ing cells, w'hich have a ladderlike 
thickening in the walls. The gall may 
seem hard if the woody cells are abun- 
dant. 

The nonparasitic bur knots, callus 
overgrowths, and infectious hairy root 
are common diseases that have been 
mistaken for the true crowm gall. 

The disease occurs the w'orld over. 
Infected nursery stock easily could have 
carried it from one place to another. 
Its economic importance varies. I?i ir- 
rigated districts and other sections with 
abundant moisture, the disease may 
occur so often that an uninfected plant 
is hard to find. A gall that develops on 
a lateral root may cause little damage. 
A gall that occurs on the main stem 
near the crr'wn and involves a consid- 
erable pan of the circumference of the 


stem, may weaken the stem, disrupt 
the flow of sap, and favor the progress 
of a cortical rot. Such a plant usually 
dies. 

Crown gall is caused by the bacte- 
rium Agrobacterium tumefacienSy a small 
Gram-negative rod. It is closely related 
to the bacteria that produce root 
nodules on leguminous plants and to 
bacteria of the colon-typhoid group. 

The crown gall bacteria grow readily 
on any of the common bacterial media. 
They do well on nitrate-sucrose-min- 
eral salt agar. 

The infection cycle is relatively 
simple. The bacteria enter the tissues 
apparently only through wounds, most 
commonly through wounds caused by 
insects or cultivation. Once inside the 
tissue, they occur primarily between 
the cells, from which they stimulate the 
surrounding cells to divide. In the 
earlier stage, that l(x>ks like the re- 
sponse to a wound, but it never heals. 
As the galls increase in size, some of the 
larger cells apparently arc crushed by 
the pressure, and the bacteria move 
into other tissues for further activity. 
The bacteria occur usually in abun- 
dance on the surface, from which they 
may be washed off and distributed by 
flowing water. Chew'ing insects may 
carry them from one plant to another 
and also may introduce them into 
wounds. Over long distances the bac- 
teria travel on the surface of nursery 
stock or in.side the tissue. Symptoms 
may not develop for several weeks, 
depending on temperature, humidity, 
and the growth of the host. They may 
not show during nursery inspection. 

Galls ordinarily develop better as the 
temperatures increase up to a certain 
point. However, on tomato, Kalanchoe, 
and certain other plants studied in 
experiments, the galls fail to develop 
much, if any, above 83° F., although 
plants and bacteria do well at the 
higher temperatures. 

Moisture, light, and mineral nutri- 
ents may influence the development of 
the galls. Frequently, since no growth 
means no gall, that merely reflects the 
growth of the plant itself. 



enOWN OAll^A M 

Various insects living in the ground 
seem important, especially those on 
raspberries. They chew on the roots 
and the galls. They open infection 
courts and may actually transmit the 
bacteria from one injury to another. 
Cultural practices likewise may be im- 
portant. Obviously a type of cultiva- 
tion that encourages insects or pro- 
duces many injuries on the roots or 
crowns may encourage infection. 

The means for combatting crown 
gall are closely tied in with the environ- 
ment and the way the disease develops. 
Perhaps one of the best control meas- 
ures is to grow a crop that is not sus- 
ceptible for several years between crops 
that are susceptible. A crop that re- 
duces the presence of root-chewing in- 
sects likewise discourages this means of 
transmission. Sometimes, if the infec- 
tion is carried on the surface of the 
planting stock, a surface disinfectant 
may be helpful — but not fully reliable, 
because in some instances the bacteria 
may enter a wound and be protected 
against the disinfectant. Such infec- 
tions are impossible to detect during 
nursery inspection because frequently 
the period of incubation is not long 
enough to permit gall development. 
Galls that develop in the nursery on 
the unions nf piece-root apple grafts 
have been controlled by special adhe- 
sive tape wrappers. They may contain 
the disinfectant, corrosive sublimate, in 
the adhesive mixture. 

While the economic importance of 
crown gall makes it a critical disease, 
particularly on sugar beets, fruits, and 
some types of nursery stock, it has still 
greater importance as a tool for work 
dealing with the fundamentals of dis- 
eased growth. Erwin F. Smith called 
crown gall a '‘plant cancer.” 

The changes from normal growth to 
diseased grow’th involve many funda- 
mental biological problems. The more 
one learns about biochemistry the 
greater appear the parallels betw'een 
plants and animals — Ix^tween cabbages 
and kings. From the standpoints of 
growth stimulation and, what is much 


kifGNANT OIOWTH 69 

more important, growth inhibition, 
many basic substances, including vari- 
ous carbohydrates, fats, proteins and 
their derivatives, mineral salts, vita- 
mins, and enzymes, are common fac- 
tors occurring both in plant and animal 
cells. Admittedly, a description of such 
fundamental work becomes a bit tech- 
nical. Its importance extends far be- 
yond the agricultural field. 

For fundamental work on growth, 
plants have certain advantages over 
animals. The plants have no complex 
nervous, digestive, and circulatory 
systems, which complicate the basic 
physiology. Large numbers of plants 
may be used at a relatively low cost. 
The possibilities for genetic purity with 
plant material arc real and important. 
Many inbred lines are available for 
use. Still better, various plants, such 
as many fruits, are ordinarily repro- 
duced by vegetative propagation. 
Thus, the different individuals are 
genetically identical. For details of 
tissue metabolism, tissue cultures from 
higher plants offer a relatively simple 
and direct approach not yet possible 
with tissue from higher animals. These 
cultures grow indefinitely upon media 
which contain only nutrients with 
known chemical formulas. Ordinarily 
the tissues grow well without a change 
of nutrient for some weeks. The grow- 
ing tissue develops in a compact mass 
easily separated from the medium. 
Thus, any change in growth may be 
determined merely by weighing the 
tissue pieces. Many ways arc available 
for inducing at will one or another 
kind of diseased growth. 

What actually initiates these dis- 
eased growths, what keeps them 
going, and especially how they can be 
inhibited are critical questions. They 
have stimulated much speculation and 
many experiments. One may approach 
this problem from the standpoint that 
the bacteria start off the diseased de- 
velopment. They may or may not be 
necessary to keep the diseased groMrth 
going. To make a comparison with 
firearms, one might consider that the 
causal agent operates as a trigger 



YEARBOOK OF AGRICULTURE 1953 


70 

mechanism to set things off. However, 
a trigger alone is not enough. The gun 
must be loaded. Also important arc 
the amount of the load, the character 
of the load, the amount of dampening 
the load carries, and so on. 

Detailed data on the metabolism of 
the plant, of the causal agent, and of 
both together are necessary. 

As the crown gall bacteria develop 
in suitable culture media, a number of 
physical and chemical changes occur. 
Knowing what happens in such media 
may help to clarify the action of the 
bacteria as they work in host tissue. 

Among the critical physico-chemical 
changes are the modification of the 
hydrogen-ion concentration, a reduced 
oxidation-reduction potential, a de- 
creased osmotic pressure, and an in- 
creased viscosity. 

Among the chemical factors arc the 
ability to use an unusually large num- 
ber of different sources of carbon and 
nitrogen. Likev^ise these bacteria tol- 
erate many kinds of inhibiting sub- 
stances. 

The metabolic products known to 
be formed by crown gall bacteria have 
thus far been surprisingly simple, 
principally carbon dioxide. No vola- 
tile organic material has been de- 
tected, By far the most common resid- 
ual metabolite is a bacterial gum. In 
culture its weight is considerably 
greater than that of the bacterial cells. 
One molecule of this gum contains 
approximately 24 glucose molecules. 
The gum is viscous, takes up moisture, 
and is chemically rather inert. Appar- 
ently neither the bacteria nor the host 
plant has an enzyme system capable 
of attacking it. 

The metabolites resulting from ni- 
trogen in the medium have received 
much less attention. Ammonia was 
one of the first products reported and 
has come the nearest to being a com- 
mon factor of any found among the 
various cell-stimulating bacteria. 

The crown gall bacteria have been 
shown to produce the vitamins biotin, 
riboflavin, nantothenic acid, and thia- 
mine. Since they grow in synthetic 


media, they produce any other such 
material needed for their metabolism. 

An analysis of large quantities of the 
crown gall bacteria has revealed the 
presence of various lipids. These are 
more or less toxic when strong prepa- 
rations are placed upon host plants. 

The attenuation of the virulent cul- 
ture by means of certain amino acids 
and related compounds has an inter- 
esting bearing upon this problem. The 
cultures were grown in media with a 
relatively alkaline reaction and only a 
tiny bit of the amino acid, glycine, but 
enough to reduce but not to stop 
growth. After a scries of 15 or more 
successive transfers made at intervals 
of several days, the cultures gradually 
lost their capacity to induce gall forma- 
tion. In some cases, if the attenuation 
was not carried through too many 
transfers, virulence was restored by a 
series of transfers on ordinary media. 
In other cases where the cultivation on 
glycine was carried several transfers 
beyond the point of attenuation, such 
restoration did not occur in 4 years. 

A re.storation of virulence has been 
accomplished also by irradiating partly 
attenuated cultures with ultraviolet 
light so as to kill all but one in a thou- 
.sand. The .survivors commonly showed 
a conspicuous increase in vinilence. 

I’he morphological responses of the 
plant ti.ssuc to crown gall bacteria show 
important changes. The wound that 
introduced the bacteria flooded the in- 
tercellular .spaces and provided a cul- 
ture medium for the bacteria. The 
cells around the bacteria enlarged 
within 2 days and the adjacent cell 
walls turned somewhat brown and took 
ordinary stains mure intensely than 
normal walls. Within 4 days new crown 
gall cells had formed. In the early 
stages of development the new cell 
walls were laid down in somewhat the 
same manner as tho.se from a wound. 

This 4-day interval was determined 
ii]^epcndently with inoculated peri- 
winkle plants in which the bacteria 
were killed by exposing the plants to 
high temperature. Plants heated after 
3 days developed only small galls, but 



CROWN OAli — A MAIIONANT GROWTH 


7 > 


those heated after 4 days went on to 
devdop galls which were free from the 
crown gdl bacteria. 

The old ideas about continuing dis- 
eased growth without the causal agent, 
once disease in the tissue was started, 
thus have received experimental evi- 
dence with periwinkle. This brings 
closer still the comparison between 
crown gall and certain diseased growths 
in animals. How frequently such au- 
tonomous growth also occurs in other 
plants remains uncertain. The failure 
of many “secondary” crown galls on 
sunflower, Paris-daisy, and marigold 
to continue development after the bac- 
teria have died has suggested caution 
about too broad inferences. 

The differences in chemical compo- 
sitions between the galls and corre- 
spondingly healthy tissue have been 
examined. Approximate analysis indi- 
cated that the gall tissues resemble 
those of young plants, being high in 
nitrogen and low in fibrous material. 
Considerable variations were observed, 
dep)cnding on the time of collection 
and on the species of plant. The ac- 
tively growing tissues contained more 
ascorbic acid. An increase of thiamine 
appeared in the gall tissue w^ithin a 
week. 

The enzyn ic content of the galls was 
different from that of the healthy tis- 
sue. The galls contained 86 and 57 
percent more than the stems of catalase 
and peroxidase on a total nitrogen 
basis. The galls had active tyrosinase, 
while the stems had little, if any. Fur- 
thermore, the galls contained rela- 
tively much more glutathione than 
the stems. 

Reduced respiration levels by the 
gall tissues have appeared important. 
With the coasiderable excess of oxidiz- 
ing enzymes, the suggestion has ap- 
peared that the basic metabolic activ- 
ity of the galls may be relatively 
anaerobic in comparison with that of 
the neighboring tissue. This condition 
deserves further study as a causal factor 
for cell stimulation. It may be corre- 
lated perhaps with the earlier observa- 
tions that the tissues involved had 


flooded intercellular spaces. The diffu- 
sion of oxygen through normal tissue 
is 2,000 to 3,000 times greater than 
that in tissue with water in the inter- 
cellular spaces. 

As the amount of hyperplastic tissue 
increases in size, more difficulty with 
gas exchange doubtless also develops. 
The crown gall bacteria lower the oxi- 
dation-reduction potentials of the ma- 
terial in which they are growing. The 
formation of ammonia and the con- 
sequent change of the pH in an alka- 
line direction also lowers the oxida- 
tion-reduction potential. The bacteria 
and gum block certain intercellular 
spaces. The presence of gummy ma- 
terials, which are hygroscopic, might 
cause the cells to swell and to metabo- 
lize more slow'ly. The reduced oxygen 
uptake in the presence of 3-indole- 
acetic acid at a gall-inducing concen- 
tration has added interest. 

Among the well-known growth sub- 
stance effects are increased epinasty, 
adventitious roots, cambial activity, 
bud inhibition, and delayed abscission. 
All were found associated with crown 
gall on tomato. The presence in galls 
has been amply demonstrated of some- 
thing like 3-indoleacetic acid at ap- 
proximately 6 to 1 2 parts per billion — 
an amount comparable to that in some 
actively growing and normal plant 
parts. While these extremely small 
amounts strikingly affect certain tis- 
sues, much stronger and almost lethal 
concentrations are needed to induce 
chemical galls. 

The stimulation of tissue about in- 
oculations with the attenuated crown 
gall bacteria has been possible not only 
with the virulent culture higher on the 
stem but also with galls induced by 
certain chemicals. However, no cor- 
relation has appeared between the for- 
mation of these chemical galls and any 
other growth substance effects. 

The po.ssibility of studying the phys- 
iology of these diseased tissues has been 
greatly enhanced by the development 
of tissue cultures. These consist of 
masses of largely undifferentiated cal- 
lus that grow indefinitely on synthetic 



YIAtBOOK OF AOilCULTUII 1983 


72 

media. Thus, science has an important 
tool to determine which substances the 
tissues can use, which are not ayailable, 
and particularly which are inhibiting. 

The diseased tissues in culture have 
been derived commonly from crown 
galls that were free from bacteria, or 
&om growths having a comparable 
origin. Extensive studies have been 
made in relation to the best physical 
conditions, the importance of plant 
and bacterial products, the influence of 
different concentrations of various 
mineral salts, the responses to the more 
common growth substances, and the 
activity of different sources of nitrogen 
and different sources of carbon — the 
latter including sugars and polysac- 
charides, alcohols, and the salts of or- 
ganic acids. Some of the common 
metabolites appear particularly impor- 
tant, either for the stimulation or in- 
hibition of growth. Likewise, the con- 
centrations of certain metabolites seem 
as important as the substances them- 
selves. 

These tissue-culture studies also have 
shown some striking differences be- 
tween the tissues from different species. 
Are they opening further the door for 
physiological as well as morphological 
understanding of tissues? In any case, 
many interesting possibilities appear 
for studying various aspects of tissue 
metabolism in health and disease, for 
clarifying the relations between host 
and pathogen, and especially for learn- 
ing more about diseased growths. 

Growth inhibitions from certain ami- 
no acids and organic acids have led 
to the hope that still other and more 
pKJwerful agents would soon be found. 
The most active inhibition came with 
analogs of pteroylglutamic acid. These 
inhibited callus growth at 10 to 100 
parts per billion. The 4-amino-N*»- 
melhyl-pteroylglutamic acid applied 
locally to young crown galls on sun- 
flower at o.i milligram per milliliter 
completely inhibited gall development. 
Certain nitrogen mustards, penicillin, 
8-azaguanine, and cortisone also have 
inhibited c;x>wn gall. 

Many trigger agents have been as- 


sociated with diseased plant growths. 
Whether they operate by providing 
stimulation or by removing inhibitors 
remains to be determined. However, 
as mentioned earlier, the trigger is not 
enough. What happens depends on the 
kind of load and the amount and 
whether it is dampened by inhibitors. 
The use of tissue cultures has opened 
the way for determining much about 
the load, about dampening materials, 
and about the amounts of both neces- 
sary for effectiveness. 

Basic information is being developed 
not only for the influence of common 
mineral salts, sources of carbon, sources 
of nitrogen, and various metabolites, 
but also for concentrations that en- 
courage or inhibit growth. 

The idea that disea.sed growth de- 
velops from a lack of balance among 
critical factors fits well into this con- 
cept. While we shall continue to ana- 
lyze individual factors that by their 
presence dr absence may change nor- 
mal into pathological growth or keep 
it going, no one thing may be responsi- 
ble. For normal growth, a number of 
factors operate in a suitable balance. 
However, in pathological growth of 
one kind a group of these factors may 
be out of balance. Likewise, in patho- 
logical growth of another kind, the 
balance may be disturbed in some 
other way. 

A. J. Riker has been professor of plant 
pathology in the University of Wisconsin 
since /pj/. He is the author of Introduc- 
tion to Research on Plant Diseases 
{with R. S. Riker) and many research re’- 
ports on bacterial plant diseases^ diseases of 
forest trees^ and factors that influence pathch 
logical growth. For 7 years he was an 
editor of Phytopathology. 

A. C. Hildebrandt is an assistant 
professor of plant pathology in the Vniver* 
sity of Wisconsin. After earning his doctorate 
at Wisconsin in /p45, he has been engaged 
in research on crown gaily mineral and 
carbohydrate metabolism of tissue cultureSy 
environmental factors affecting plant tissue 
culture growthy vitaminSy and growth^ 
regulating substances. 



BROOMRAFES, DODDERS, AND MISTLETOES 


Broomrapes, 
Dodders, and 
Mistletoes 


Lak€ S. cm 

Parasites are plants and animals that 
get their living at the expense of other 
organisms, which are called the hosts. 

Usually we think of parasites as 
lower forms of life, such as fungi and 
bacteria. A number of flowering, or 
seed plants, however, are parasitic on 
vegetation. Parasitism appears in vary- 
ing degrees in several widely separated 
botanical families and is regarded gen- 
erally as a degenerative process in 
species that once were free-living. A 
higher plant that has become parasitic 
does not, as far as wc know, return to 
independence. 

The parasite depends physiologically 
for its existence on the host plant. The 
host may incidentally supply protec- 
tion or physical support, although 
plants that derive only those benefits 
are not classed as parasites but are 
called epiphytes. Examples of epiphytes 
are Spanish-moss and some tropical or- 
chids that attach themselves to trees 
but are physiologically independent 
even though they maintain no contact 
with the soil. 

The parasitic seed plants vary widely 
in their dependence. The more inde- 
pendent ones are referred to as hemi- 
parasites, half-parasites, or water- 
parasites. All of them produce chloro- 
phyll and therefore are capable of 
manufacturing food from carbon 
dioxide and water, although they 
depend on the host for certain dis- 
solved minerals and perhaps organic 
substances. Some absorb from their 


73 

hosts all the water iliey need for tran- 
spiration and the manufacture of food. 

The mildest hcmipara.sites look like 
normal green plants growing in soil. 
They steal their food through root con- 
nections, often inconspicuous, from 
surrounding host plants, which may 
suffer little or no harm from the 
arrangement. Notable parasites of this 
type are the sandalwood tree of India 
and its near relative Commandra (falsc- 
toadflax), a common herb in North 
America. The mistletoes illustrate the 
opposite extremes of this group, having 
lost contact with the soil and being 
dependent on their hosts for all water 
and dissolved minerals even though 
the chlorophyll-containing species can 
manufacture sugars and starches in 
their green leaves and stems. 

The complete parasites have no 
chlorophyll and therefore depend 
wholly on their hosts for nourishment. 
Plants that have degenerated to this 
extent are never green. Their leaves 
usually are reduced to inconspicuous 
scales and they exhibit a marked modi- 
fication of their functional root system, 
which develops entirely inside the host 
tissues. 

Parasitic seed plants important to 
agriculture, forestry, and arboriculture 
in North America are relatively few 
and can be classified broadly in three 
groups: (i) Mistletoes are green, yel- 
lowish, or brownish plants growing on 
the stems and branches of trees or 
shrubs. (2) Dodders are slender, twin- 
ing, orange or yellowish, rarely white 
or purplish, leafless, threadlike stems 
often forming dense tangled mats over 
host plants. (3) Broomrapes are clumps 
of whitish, yellowish, brownish, or 
purplish stems that arise from the roots 
of host plants. 

The n.\me mistletoe is derived 
from the Saxon mistl-tariy meaning 
“different twig’* an indication that 
the ancients recognized it as something 
apart from the branches of the host 
tree. It was featured in the Greek, 
Norse, and Germanic legends as a 
plant vested with supernatural pow- 



YEARBOOK OF AGRICULTURE 1953 


74 

ers for good and evil. The Druids and 
other pagan peoples of Europe used 
it as a sacred emblem in their religious 
rites. The herbalists of the early 
Christian era claimed that mistletoe 
was once a forest tree but became 
dwarfed out of shame when its wood 
was used to make the cross at Calvary. 
They called it guidhel or all heal and 
prescribed it as an antidote for poisons 
and cure for falling sickness and 
epilepsy. Amulets made of the plant 
were often worn to ward off disease. 

The early American settlers consid- 
ered our leafy mistletoe of the East 
to be identical with the Viscum of 
their homeland and thus it, too, be- 
came vested with much of the tradi- 
tional symbolism that had developed 
about the European plant. Today it 
is highly prized for Christmas deco- 
rations and during the holiday pci iod 
it may shed blessings on those who 
stand beneath it. 

About 305 B. C., Theophrastus, 
the Greek botanist, recorded technical 
observations which indicated that he 
recognized mistletoe as a parasitic 
plant. In the eighteenth century, 
Carolus Linnaeus, the great Swedish 
botanist, described and named the 
principal European species Viscum 
album. The name was doubtless .se- 
lected because the w^hite berries of the 
plant were then used in the manufac- 
ture of bird lime. Strictly speaking, 
this plant is the true mistletoe, al- 
though today the name is loosely 
applied to many members of the 
botanical family Loranthaceae, which 
embraces about 900 species, mostly 
tropical tree-inhabiting hemiparasites. 
About 35 species are known in tem- 
perate North America. Five of these 
are in the genus Arceuthohium^ also 
called dwarfmistletoe; the remainder 
are in the genus Phoradendron.^ often 
called leafy mistletoe, true mistletoe, 
or Christmas mistletoe. The latter 
genus was set apart from the Old 
World Viscum in 1847 by Thomas 
Nuttall, first director of the Harvard 
Botanical Gardens; the Greek origin 
of the name means “tree thief.” 


The Phoradendrons attack chiefly 
broadlcaved trees, although in the far 
West certain conifers, especially juniper 
and its near relatives, are common 
hosts. 

All species are regarded as half- 
parasites that produce clusters of 
green, perennial. Jointed stems on the 
branches and trunks of trees or large 
shrubs. The stems mostly bear con- 
spicuous deep green, leathery leaves, 
which persist for several seasons. A few 
species, notably some of those on 
conifers and desert plants in the West, 
arc virtually leafless. 

The stems are supplied from an 
absorbing system, which develops in 
the bark and wood of the host and 
takes water and whatever else may 
be required to supplement the food 
manufactured in the aerial green 
parts of the parasite. The flowers are 
dioecious; that is, the staminate or 
male and the pistillate or female 
flowers occur on .separate plants. They 
are borne at the base of the leaves 
where ('among pistillate plants only) 
the familiar translucent, whitish ber- 
ries develop in the late fall or early 
wdnter. In some western species the 
berries are straw to pinkish in color 
and mature in midwinter. 

Within the tough outer coat of the 
berry is a single seed, which is embed- 
ded in viscin, a sticky substance. Birds 
feed on the sticky pulp of the berries. 
Some of the discarded seeds stick to 
their bills, feet, and other parts of the 
body. Thus the seeds are carried to 
other trees or other parts of the same 
tree, where they may be brushed off 
on a branch or twig, germinate, and 
perhaps develop into a new mistletoe 
plant. If the berries are left undi.s- 
turbed they disintegrate and release 
the sticky seeds, which may fall and 
adhere to a limb or twig below. Seeds 
that pass through the digestive tracts 
of birds may also germinate and start 
new mistletoe plants. 

In most parts of the country the leafy 
mistletoes are relatively scarce and arc 
regarded more as botanical curiosities 
than as serious pests. Because of their 



BROOMtAFES, DODDERS, AND MISTLETOES 


75 


value as holiday decorations, some in- 
terest has been shown in their culture 
but thus far there have been no practical 
developments in that held. Most of the 
mistletoe for the Christmas trade is 
gathered in the forests of the Southern 
and Southwestern States where it is 
abundant in some localities and where 
it provides oiT-season income for agri- 
cultural workers. The berries of Phora- 
dendron Havescens are a recognized source 
of a pressor compound (parahydroxy- 
phenylethyl amine) of limited pharma- 
ceutical value. 

In certain arid or semiarid places in 
the West, notably in Texas, New Mex- 
ico, Arizona, and California, mistletoe 
has become so abundant as to warrant 
control measures for the preservation 
of shade and horticultural trees. The 
natural woodland species that arc most 
heavily attacked — notably juniper and 
mesquite — generally have such low 
economic value that large-scale con- 
trol operations cannot be justified ex- 
cept possibly in public parks where the 
primary aim is to presei-vc natural 
vegetation for the enjoyment of gener- 
ations to come. In some parts of the 
Midwest, such hardwoods as walnut 
and elm arc sometimes severely para- 
sitized by mistletoe. 

The spread of mistletoe can be re- 
duced somewhat by breaking off the 
pistillate shoots. That must be done 
periodically because it docs not destroy 
the absorbing system from which the 
shoots develop; it merely discourages 
the parasite from producing berries, 
which may spread about the infected 
tree or to other host plants. It is practi- 
cal only if little mistletoe is present and 
the danger of reinfection from outside 
is remote. 

When infections are moderate, one 
can prune off the affected limbs and 
thus free the host plant entirely of the 
parasite. Sometimes severe trunk in- 
fections can be destroyed by removing 
the invaded bark. If hopelessly infected 
trees occur near others worthy of pres- 
ervation, it would perhaps be best to 
remove the infected ones entirely. 

Although a single species of Phoraden- 


dron may attack a variety of trees, 
strong host affinities arc usually devel- 
oped within a species in a given region. 
The extent to which crossing from one 
host species to another occurs in na- 
ture is not known exactly, although 
there are strong indications it is quite 
limited. In regions where mistletoe is a 
serious pest, it would seem advisable to 
favor tree species that are naturally 
immune or highly resistant to attack in 
that particular area even though they 
are congenial hosts elsewhere. 

The dwarfmistletoes are found only 
on conifers. In North America they do 
not attack juniper and related species, 
which are common hosts of Phoradendron. 
The mistletoe shoots themselves vary 
in color from green to yellow or brown. 
Less conspicuous than those of the 
true mistletoes, they range from tiny 
scattered outgrowths about one-fourth 
inch high tc coarse, jointed stems up to 
1 2 inches long. The seeds are borne in 
explosive, berrylike fruits. At maturity 
they may be shot 50 or more feet away 
from the host tree and thus gradually 
encroach on the surrounding forest. 
The seeds arc covered with sticky 
viscin and adhere to any surface on 
which they may alight. 

New infections are most likely to 
develop when the seeds germinate on 
young twigs of a suitable host plant. 
In that case an absorbing root pene- 
trates the tender bark and develops 
a system of strands that attack the 
living phloem of the host. They ab- 
sorb from it what is needed for the 
development of the parasite. Some of 
the strands reach the cambium layer 
of the host and are permanently em- 
bedded in the wood as it is laid down 
each year. They retain their connec- 
tion with the strands in the phloem 
indefinitely and are Called sinkers. 
After the absorbing system is well es- 
tablished, it produces buds from w^hich 
shoots develop. That may occur the 
year after infection or it may be de- 
layed for a decade or more. Flowers 
are borne on the shoots and, like 
Phoradendron^ arc dioecious. Insects 
carry the pollen from the male to the 



YEAt»OOK OF AGtlCULTUIE 1*S3 


76 

female flowers and the fruits mature 5 
to 16 months later. 

Forests heavily infected with dwarf- 
mistletoe do not produce top yields. 
The greatest losses occur in the far 
West. One species {Arceuthobium pusil- 
him) is recognized as a serious agent 
of disease in black spruce {Picea 
mariana), especially in the Lake States. 
The greatest economic damage occurs 
in ponderosa pine (Pinus ponderosa)^ 
lodgcpolc pine {P, contoria), and west- 
ern larch (Larix occidentalis) . The 
effects of infection of the host plants 
include premature death, rediiced 
growth, poor seed production, poor 
form, low quality of wood products, 
and increased susceptibility to attack 
by insects and other diseases. 

Physical removal of the parasite is 
the only known means of controlling 
it. In heavily infected stands it is 
usually necessary to depart from nor- 
mal forest management practices to 
obtain adequate protection for the 
future forest — exceptionally heavy har- 
vest cuttings, followed by stand im- 
provement designed to reduce mistle- 
toe in the unmerchantable trees. At 
least one cleaning subsequent to the 
initial eradication will be desirable 
to remove the infections that origi- 
nated before control work was started 
but which were invisible at the time. 
Economic justification for control will 
depend on the value of the stand 
attacked. A knowledge of the biology 
of the particular mistletoe species 
involved will aid in developing the 
most efficient control techniques. 

Dodder belongs to the genus 
Cuscuta^ a close relative of the familiar 
morning-glories. It is sometimes re- 
ferred to as love vine, strangleweed, 
dcvil’s-guts, goldthread, pull-down, 
devirs-ringlet, hellbinc, hairweed, 
devirs-hair, and hailweed. It com- 
monly apficars as dense tangles of 
leafless, orange or yellow strands on 
its host plants. Sometimes it is tinged 
with red or purple. Occasionally it is 
almost white. The strands develop 
from seed, v%hich germinate on the 


soil. The leafless yellowish stem gropes 
in the air until it makes contact with 
a host plant. The contact is made Arm 
by one or more coils about the host 
stem after which haustoria — the ab- 
sorbing organs — are produced. The 
haustoria invade the host tissues and 
absorb the food required for the 
dodder to continue its twining growth. 
The basal part of the parasite soon 
shrivels away, so that no connection 
with the soil remains. Growth con- 
tinues with the aid of more and more 
haustoria, wffiich arc produced at 
intervals as the stem elongates. From 
the original host plant, the twining 
stems reach out to attack others in the 
vicinity, so that a single dodder plant 
may parasitize several hosts simul- 
taneously. Many minute flow’crs occur 
in clusters on the sterns; they develop 
tiny seeds, which fall to the ground at 
maturity. The seed may remain viable 
without germinating for 5 years. 

Of the loo-odd recognized sp)ccics 
of Cuscuta^ ^2 arc reported as being 
native to the United States; 18 others 
have been introduced. The parasite 
is particularly troublesome in regions 
where clover and alfalfa arc grown 
extensively, although it is not to be 
feared in America to the same degree 
as in .some European countries, where 
the production of clover seed has been 
abandoned because of its ravages. 

Dodder also attacks flax, .sugar beets, 
and onions. A large number of both 
cultivated and wild plants also are 
its hosts, but little or no economic loss 
results from the association. Cereal 
plants arc never attacked. 

Dodder is most frequently carried to 
a farm in impure seed. It may also be 
carried in hay, manure, and irrigation 
water and on vehicles and animals. 
Prevention is therefore the first prin- 
ciple of control. Once established, it 
usually appears in small scattered 
patches. Gutting the patches belbre 
the .seed matures will often eliminate 
the para.siie. After the seed has ma- 
tured it will be nece.ssary to burn the 
infested area. Heavily infested fields 
should be plowed under or used for 



BROOMRAPiS, DODDERS, AND MISTLETOES 


hay; the stubble should be grazed 
closely, preferably by sheep. The safest 
method is to cut the crop and burn it 
in place when it dries. If conditions 
permit, an infested field can be planted 
to immune or resistant crops such as 
cereals, corn, soybeans, velvetbeans, 
or cowpeas. Chemical control is feasi- 
ble but is not recommended because 
of the high cost. 

The name droomrape is generally 
applied to the genera Orobanche and 
Phelipaea. The go-some species are 
mostly in the temperate parts of the 
world. Both belong to the botanical 
family Orobanchaceac, all members 
of which are complete root parasites. 
The family also includes the genus 
Epijagus^ commonly known as beech- 
drops, which appears under beech 
trees. From an evolutionary stand- 
point, the group is closely related to 
the snapdragons and foxgloves (Scro- 
phulariaceae). Some species of the 
parasites produce just as showy flowers 
as their independent relatives. Several 
members of the Scrophulariaceae ex- 
hibit mild degrees of root parasitism 
similar to that previously mentioned 
in the case of sandalw^ood and false- 
toadfiax. 

The broonirapes appjcar as clumps 
of whitish, yellowish, brownish, or 
purplish annual stems arising from the 
ground at or near the base of their 
host plants. The stems are 6 to i8 
inches high. They have bractlike leaves 
and numerous showy flowers somewhat 
like those of the snapdragon. An 
abundance of minute seeds is pro- 
duced in capsular fruits. The seeds 
germinate in the soil if the fibrous 
roots of a congenial host plant arc 
nearby. Otherwise they remain dor- 
mant until they lose their viability, 
which can be retained as long as 13 
years. When the primary root, or 
radicle, of a germinating seed makes 
contact with a fibrous root of its host, 
it forms a nodule of tissue, which 
becomes fused with tissues of the host. 
New roots and a stem of the parasite 
develop at that point. The stem emer- 


77 

ges while the roots form new contacts 
with other host roots at each of which 
new roots and stems of the parasite are 
produced. 

About 1 6 species of broomrape are 
regarded as pests of many crop plants. 
Three are found in the United States. 
The damage caused by broomrape in 
America does not approach the situa- 
tion in Europe. Our most troublesome 
species, hemp broomrape {Orobanche 
ramosa)^ w'as perhaps introduced on 
hempseed from China or Japan. It 
attacks a number of unrelated crops, 
but it is serious only on our hemp. A 
native species, Louisiana broomrape 
(O. ludoviciana)^ sometimes damages 
tobacco. Beyond these, the losses 
cau.sed by broomrape are negligible 
and sporadic. 

Clean seed is the best protection 
against broomrape in hemp culture. 
Tht. .seeds of the parasite can be sep- 
arated mechanically. Contaminated 
lots may be treated with hot w ater or a 
strong bluestone solution, both of 
which kill the parasite without damag- 
ing the hemp. Rotation wdth immune 
or resistant crops is perhaps the most 
practical procedure in heavily in- 
fested fields, but it should be remem- 
bered that the seeds of the para.site 
remain viable in the soil for many 
years. Small infections of broomrape 
can usually be eliminated by destroy- 
ing the aerial stems before the seeds 
ripen. 

Lake S. Gill, senior pathologist of 
the division of forest pathology. Bureau of 
Plant Industry, Soils, and Agricultural 
Engineering, is an authority on the dis~ 
eases of forest Uees in the Southwest and 
the Rocky Mountains. He \ias made com-- 
prehensive studies of mistletoe and devel- 
oped methods for its control. He is stationed 
in Albuquerque, N. Mex, He contributed 
Arceuthobium in the United States'^ to 
Transactions of the, ConnectkuJt Academy 
of Arts and Science {volume js, pages in- 
245, 1935)- ^'ith J. L. Bedwell he wrote 
Dwarf Mistletoes^'" for Trees, the Year- 
book of Agriculture 1949 {pages 458-461), 



78 


YEARBOOK OF AORICUITURE 1953 


The Tiny but 

Destructive 

Nematodes 


Albert L, Taylor 

Nematodes differ from most of the 
other organisms that cause plant dis- 
eases in that they belong to the animal 
kingdom, not the plant kingdom. 

The plant parasitic nematodes, or 
eelworms, are representatives of a 
large group of species that the zoolo- 
gist considers quite different from the 
other kinds of animals called worms. 
That is, they are not closely related to 
the earthworms, flat worms, wire- 
worms, grubs, and cutworms, but are 
in a class apart. They have no close 
relatives. 

Several thousands of species of nem- 
atodes are known. They differ in form, 
habits, and habitat. Some are para- 
sites of animals and of man. Others 
live in the fresh waters of rivers, ponds, 
and lakes. Many live in the salt waters 
of the sea. A great number live in the 
soil. Most of those livin^g in the soil 
can be classed as harmless and some 
even as distinctly beneficial, but sev- 
eral hundred species are known to 
feed on living plants as parasites and 
to be the causes of a variety of plant 
diseases. 

Plant parasitic and free-living nem- 
atodes occur in enormous numlDers in 
all kinds of soil in which plants can 
grow. A single acre of cultivated soil 
may contain hundreds of millions, but 
they are seldom if ever seen, even by 
the farmer who is constantly working 
with the soil. The reason is simply 
that, although they are thousands of 
times larger than bacteria, they arc 


just a little too small to be easily seen 
with the naked eye, even when sep- 
arated from the soil. The length of 
the full-grown plant parasitic nema- 
tode may be less than one sixty-fourth 
of an inch and seldom exceeds one- 
eighth inch. 

Most are very slender, as the name 
eel worm suggests. Nevertheless, they 
have a highly complex organization. 
Their small bodies have muscular sys- 
tems, specialized organs for feeding, a 
digestive system, a nervous system, an 
excretory system, and a well-developed 
reproductive system. Both males and 
females occur in most species, but 
reproduction without the males is 
not unusual. 

The life history of plant parasitic, 
nematodes is simple enough. Eggs may 
be deposited in the soil or in the plant 
on which the female feeds. In the eggs 
the immature forms, the larv'^ae, de- 
velop and eventually hatch. If plants 
on which they can feed are available, 
they may begin to feed immediately, 
developing through several distinct 
stages. At the end of each of these, a 
molt takes place. After the last molt, 
the nematode becomes sexually ma- 
ture and able to reproduce. 

■Most of the forms that have been 
closely studied have a minimum length 
of life cycle, from egg to egg-laying 
female, of several days to sever^ 
weeks. The maximum time may be 
much longer, as sexual maturity is not 
reached until the nematode begins to 
feed on a living plant. Until then it 
remains in the larval stage and lives 
on a reserve supply of food originally 
derived from the egg. The length of 
time this reserve foc^ supply lasts de- 
pends on circumstances. In warm, 
damp soil, the nematode will be very 
active and use it up in a few weeks or 
months. In cool or dry soil, activity is 
less and the food supply lasts longer. 
The species found in cold climates 
can easily live over winter and arc 
not killed by freezing of the soil. Some 
species are killed if subjected to dry- 
ing, but others enter a dormant state 
in which they can remain alive for 



THI TINY BUT DCSTIUCTIVE NEMATODES 


yean and from which they can revive 
in a short time if moistened. The most 
remarkable of these are parasites of 
wheat, rye, and other grains and grass- 
es. The wheat nemat^e has been re- 
vived after dry storage for 28 years. A 
species that parasitizes rye has been 
revived after 39 years. Certain species 
can live a long time outside in moist 
soil. Females of the golden nematode 
of potatoes and its relatives become 
transformed at death into highly dur- 
able cysts. Because few or none of their 
eggs are deposited, the cysts contain 
eggs with unhatched larvae, which 
may remain alive for i o years or more. 

As plant parasitic nematodes neither 
feed nor reproduce except on living 
plants, survival of the individuzd de- 
pends on its reaching a plant on which 
it can feed before its reserve food sup- 
ply is finally exhausted. Unless hatched 
in a plant, that means that the nema- 
tode must travel through the soil in 
search of food. This movement seems 
to be more or less random wandering 
and, since nematodes are small, is 
confined to a small area of soil. Perhaps 
most nematodes never get more than a 
foot or two from the spot where they 
were hatched. Therefore nematodes 
spread very slowly by their own efforts. 

Plant parasitic nematodes have many 
enemies in the soil. At any stage of 
their life they may be captured and 
devoured by other soil animals, such 
as insects or predatory free-living 
nematodes. Certain soil fungi have 
traps that seem especially designed to 
catch nematodes. Some are loops 
which close when a nematode starts to 
crawl through. Others have sticky sur- 
faces to which nematodes adhere. In 
either instance, the fungus grows into 
the body of the nematode and kills it. 

Even though the accidents of life 
take a large toil of the plant parasitic 
nematodes, a field population is seldom 
exterminated. Like other parasites, the 
nematodes manage to reproduce just a 
little faster than they can be wiped out. 
A single female root knot nematode 
may produce more than 500 eggs. If 
only a few of them survive to reprc^uce 


79 

in turn, a great increase in the popula- 
tion of a field can take place during a 
summer when several generations fol- 
low one another. 

Information accumulated during the 
past century indicates that all of the 
crop and ornamental plants grown in 
the world can be attacked by plant 
parasitic nematodes. If there are ex- 
ceptions to this rule, they must be very 
few, indeed. Probably most weeds and 
wild plants are also attacked. 

That does not imply that any species 
of plant parasitic nematode can attack 
any kind of plant. All plant parasitic 
nematodes are more or less specialized, 
attacking some plants freely and others 
not at all, even when given every op- 
portunity to do so and when no other 
source of food is available. So far as a 
given species of nematode is concerned, 
different kinds of plants may have 
varying degrees of suitability as food. 
On some they will not attempt to feed. 
On others they will feed, but seem un- 
able to reproduce. Such plants are 
called immune to the nematode species 
concerned. On other plants reproduc- 
tion is inhibited to various degrees; 
they are called resistant. Plants on 
which normal reproduction takes place 
are called susceptible. It should be 
emphasized that any such clzissifica- 
tion of plants can be taken as applying 
to only a single species of nematode. 
Plants immune to attack by one species 
of nematode may be highly susceptible 
to attack by others. 

The species range from highly spe- 
cialized (those that attack only a few 
kinds of plants) to polyphagous (those 
that attack a great many different 
kinds of plants) The reasons therefor 
are not known. In practice there seems 
to be no way of knowing which plants 
might be attacked by a given species 
except by experiment. Resistance to 
nematodes sometimes can be found in 
horticultural varieties of crops or in 
other species of the same plant genus. 
Advantage is taken of this fact in the 
development of nematode-resistant 
crop varieties. 

Plants almost invariably become in- 



8o 


YEARBOOK OF AGRICULTURE 1953 


fected by nematodes that move into 
them fi'om the soil. As would be ex- 
pected, the undergrounc} parts of 
plants, roots, tubers, corms, and rhi- 
zomes are more apt to be infected than 
above-ground parts. Infection of stems, 
leaves, and flower parts is fairly 
common, however. 

Damage to plants attacked l)y nem- 
atodes is due primarily to the feeding 
of the nematodes on- the plant tissues. 
All the important plant parasitic nema- 
tode species have a special feeding 
organ, known as a stylet or spear. As 
seen in profile under the high-power 
microscope, the typical stylet resembles 
a nail with a thickened head, although 
close examination reveals that the head 
is composed of three more or less dis- 
tinct knobs. Stylets, highly variable in 
size and shape according to the species 
of nematode, range from comparatively 
long to very short, and the knobs have 
a variety of forms, ranging from large 
and distinct to nearly absent. Those 
differences are useful in the identifica- 
tion of genera and species. The stylet 
really is more like a hypodermic needle 
than a nail. It is hollow, and the nema- 
tode uses it to pierce plant tissue or cell 
W'alls. With the stylet pushed into a 
cell, the nematode can suck out the 
cell contents. In preparation for this, 
it may inject a digestive secretion into 
the cell, evidently to liquefy and partly 
digest the food before it is ingested. 
Nematodes may enter the plant to 
feed, may feed from the outside, or be 
only partially embedded. Feeding hab- 
its vary according to species. 

The feeding of a nematode may kill 
the cell or may simply interfere with 
its normal functioning. If the cell is 
killed, it often is quickly invaded by 
bacteria or fungi. If the cell is not 
killed, it and the adjacent cells may be 
stimulated to enlarge or multiply. 
Con-sequently the most common typjes 
of nematode damage arc manifest as 
rotting of the attacked parts and ad- 
jacent tLssue or the development of 
galls and other abnormal growths. 
Either can interfere with the orderly 
development of the plant and cause 


shortening of stems or roots, twisting, 
crinkling or death of parts of stems and 
leaves, and other abnormalities. 

Those symptoms often arc compli- 
cated by the presence of secondary 
invaders in the affected parts and, 
particularly in advanced cases, may 
present a confusing picture. Conse- 
quently the specialist depjends only 
partly on symptoms for diagnosis and 
searches for nematodes in the plant or 
in the nearby soil. 

The following representative nem- 
atode diseases are common and can be 
recognized fairly easily. It should be 
remembered that it is easy to mistake 
nematode dlserises for those caused by 
some other organisms, and vice versa. 
Merely finding nematodes in diseased 
plant tissue or the soil is not conclusive 
evidence that they arc the cause of the 
trouble. Nonparasitic types of nema- 
todes often are found in great numbers 
in decaying plants and the soil always 
contains a variety of free-living nema- 
todes. Positive identification should 
always be obtained before starting ex- 
pensive or troublesome control meas- 
ures. On the other hand, nematodes 
should alw^ays be considered as a pos- 
sible cause of plant diseases when root 
systems arc galled, shortened, or re- 
duced by rotting; when the stems are 
shortened and thickened and the leaves 
do not grow normally; and some other 
abnormal growth is noted. 

Probably the easiest of the nematode 
diseases to recognize is root knot, 
caused by nematodes of the various 
species of the genus Addoidogync (for- 
merly grouped under the name Hetero- 
dera marioni). They are called root knot 
nematodes. As the name implies, they 
cause the formation of knots or galls 
on roots of a great variety of crop and 
ornamental plants, including trees and 
shrubs. 'Fhe typical simple galls arc 
best observed on the younger roots, 
where they may look like beads on a 
string. Galls caused by at least one 
species of the genus commonly have 
several short, adventitious roots that 
rise from the upper part and produce 
a bushy appearance of the root. 



THE TINY BUT DESTRUCTIVE NEMATODES 


8l 


On larger roots, compound galls 
may be an inch cm* more in diameter. 
Severely infected roots have a rough, 
clubbed appearance. Often there is 
considerable rotting of the roots, 
particularly late in the season. Galls 
may also be formed on tubers and on 
parts of the stem in contact with the 
soil. Positive identification of the galls 
is made by breaking them open and 
looking for the nematodes; usually 
the adult females arc found. They 
arc pear-shaped and not cel-slii^ped, 
like most nematodes. They are pearly 
white and about as big around as 
the shank of a common pin. That is, 
they are large enough to be seen with 
the naked eye and quite easy to see 
with a magnifying glass, particularly 
in a portion of the root that has begun 
to rot so that they are in contrast 
with the brown root tissue. The egg 
masses of the root knot nematode 
are also fairly easy to see. These 
arc brown in color, often as large as 
the nematodes, and are found cling- 
ing to the side of roots. When lifted 
off, the female will be found embedded 
in the root tissue. A microscope allows 
one to see the eggs in various stages 
of development and the hatched 
larvae in the egg mass. Sometimes 
the male, a slender worm quite 
different from the female, can also 
be found in the egg mass. 

Nematodes of the genus PratyUnchus^ 
known as meadow or root lesion 
nematodes, are another common type 
of nx)t parasites. They feed in the 
cortex of roots and destroy the cells 
on which they feed. Fungi then attack 
the dead tissue. In the early stages 
the only visible symptom of attack is 
a small, reddish-brown lesion on the 
root. The lesion later enlarges, often 
girdles the root, and eventually severs 
it. Heavily attacked plants have 
greatjj^, reduced root systems; most of 
the feeder roots are destroyed late in 
the season. The same sort of damage 
is also caused by other nematodes and 
by other types of soil organisms ; positive 
diagnosis depends therefore on iden- 
tifying the meadow nematode. 


Stubby root nematodes (species of 
the genus Trichodorus) and sting nema- 
todes {Belonol&imus gracilis) are external 
parasites, which apparently feed most- 
ly on root tips. The feeding causes the 
root tip to stop growing and turn 
brown. Parts of the root may then die, 
probably l.)ecause of attacks of second- 
ary invaders. The final result is a re- 
duced root system with many .short 
root stubs. The attacks are particularly 
damaging to seedlings. Being external 
parasites, these nematodes will be 
found only in the soil. 

Bulb and stem nematodes, species 
of the genus DitylenchuSy cause more 
or less localized deformations of stems 
and leaves. Stems are shortened and 
thickened; leaves are twisted, short- 
ened, and otherwise distorted; and 
bulbs, such as narcissus and onion, 
become soft. In the later stages there 
may be rolling of the infected tissues. 
Nematodes can be found in large 
numbers in the affected parts, but are 
very slender and difficult to see with- 
out a microscope. 

The wheat nematode, Anguina trilici, 
causes deformation of leaves of wheat 
and other grains in the early stages 
of growth. The nematode later in- 
vades the developing car, causing the 
formation of galls in place of grain. 
The galls are shorter than normal 
wheat grains and look much like smut 
balls. But smut balls are soft enough 
to crush with the fingers and nematode 
galls arc hard. If the gall is cut open 
and placed in a little water, the con- 
tents spill out and can be seen under 
the microscope to be thousands of 
minute worms, the larvae of the 
nematode. If still alive, they will 
start active movement in a few hours. 

The various kinds of nematode 
damage interfere with the growth of 
plants. Reduction in the size of the 
root system by rotting or galling 
restricts its efficiency in obtaining 
the food and water the plant must 
get from the soil. 

Root knot galls distort the tissue that 
has the function of conducting food 



82 


YEARBOOK OP AGRICULTURE 1953 


and nutrients to the upper part of the 
plant. Damage to stems and leaves also 
interferes with normal growth. Conse- 
quently the yield of crop plants is re- 
duced. Crippled plants cannot produce 
a high-quality crop. With some crops, 
such as carrots and white potatoes, 
galls and rot caused by nematodes can 
make culls out of what would other- 
wise be salable produce. 

The general appearance of a crop 
heavily attacked by nematodes that 
damage roots gives the impression that 
it is suffering from lack of fertilizer and 
water, even when they are available in 
the soil in abundance. The color is a 
lighter or more yellowish green than 
normal. Nematodes are seldom evenly 
distributed in the soil, so the growth of 
plants is uneven and patches of stunted 
plants appear here and there. Heavily 
infected plants may die prematurely, 
because of rotting of the roots, while 
clean or lightly infected plants arc still 
growing normally. 

The relation of nematodes to other 
soil-borne diseases, such as fusarium 
wilt of cotton, is not clearly defined. It 
is certain that plants attacked by both 
nematodes and bacterial or fungus dis- 
eases often suffer severe injury and that 
control of nematodes has often resulted 
in fair control of the other disease. 
The usual theory is that nematodes, by 
damaging the plant tissue, prepare the 
way for infections by bacteria and 
fungi, which would not occur other- 
wise, but it has been difficult to 
demonstrate this relationship experi- 
mentally. 

I have no reliable estimates of the 
amount of damage nematodes do to 
crops in the United States each year, 
but there is general agreement that it 
is at least several hundred million 
dollars. The use of soil fumigants for 
nematode control during the past sev- 
eral years has often produced dra- 
matic proof that nematodes in the soil 
can make the difference between a 
good crop and one not worth harvest- 
ing. Yield increases of 25 percent to 
50 percent after soil fumigation arc 
common. Experiments with soil fumi- 


gation have also made it evident that 
severe nematode damage can occur in 
any part of the United States on a 
great variety of crops, including tree 
crops. It is also evident that severe 
nematode damage is not confined to 
the farm. Home gardens and orna- 
mental plantings in city yards also are 
often damaged; there may even be 
damage to the flower pots on the win- 
dow sill. New kinds of plant parasitic 
nematodes are constantly being found 
as aq^ new locations for the more fa- 
miliar species. 

Little is known of the origins of the 
plant parasitic nematodes, but infor- 
mation as to their distribution in 1953 
indicates that man has been largely 
responsible for their multiplication and 
spread from place to place. Being so 
very small, they are often unnoticed 
contaminants of plants, roots, bulbs, 
and tubers used for planting. It is 
probable that some of the worst of the 
nematode pests have moved from 
country td country around the world 
with such material. Soil moved from 
place to place, purposely or accident- 
ally, may also contain plant parasitic 
nematodes. They arc transported over 
long and short distances in soil adher- 
ing to farm implements and vehicles or 
to the feet of men and animals. Drain- 
age water carries them from field to 
field. Certain species may be blown 
about by the wind. 

After plant parasitic nematodes are 
introduced into a field, it may be a 
long time before their presence is 
noted. This is partly because increase 
from a small number is a slow process. 
jEven when some damage to the crop 
is noted, the trouble may be atuributed 
to declining soil fertility or to a suc- 
cession of unfavorable growing seasons. 

Albert L. Taylor is a member of the 
division of nematology investigatiomfOf the 
Bureau of Plant Industry^ Soils, and Agri>^ 
cultural Engineering. He has done experi- 
mental work on soil fumigation and, while 
employed by the Shell Chemical Corporation 
from to ig^, did research and develop- 
ment work on the soil fumigant D-D, 



THi IFFiCT OF WEATHEI ON DISEASES 


83 


The Effect of 
Weather on 
Diseases 

Paul R. Miller 

Three things must happen at exactly 
the same time if an infectious plant 
disease is to occur. One, a susceptible 
plant must be in a vulnerable state. 
Two, the parasite that causes the 
disease must be in an infective stage. 
Three, environmental conditions must 
be favorable for disease development. 

The environment of a plant consists 
of the air around it and the soil in 
which it grows. The environment of a 
parasite alternates between the body of 
its host and either air or soil, according 
to whether it attacks plants above or 
below ground. 

The aerial environment actually is 
the weather. It is made up of light, 
temperature, snow, rain, atmospheric 
humidity, dew, cloudiness, sunshine, 
wind, air currents, evaporation, and 
atmospheric pressure. Perhaps each 
clement of weather affects the occur- 
rence of disease in some way, but 
temperature and moisture apparently 
are the limiting factors for most diseases. 

The amount of heat and moisture 
available in the soil environment de- 
pends on weather (or on irrigation), 
together with the ability of individual 
soil types to absorb and retain heat 
and moisture. 

The surface of the soil affects the air 
in contact with it and a short distance 
above it. The state of the atmosphere 
near the ground, therefore, is different 
from that higher up. Depending on 
topography, the direction of exposure, 
the color, type, and moisture content 


of the soil, the amount and kind of 
plant cover, and other circumstances, 
entirely different environments — mi- 
croclimates — can exist close to each 
other. As all plants live wholly or 
partly within this lowest zone, the 
weather in it affects them and their 
diseases. 

The weather of any one region fluc- 
tuates over a rather definite range and 
averages up into a characteristic cli- 
mate. For any given period, however, 
weather may not conform to the re- 
gional climate at all. That is why 
forecasts are indispensable to any 
undertaking that is greatly influenced 
by weather conditions. If weatVier 
progressed exactly according to climat- 
ic specifications, forecasts would not 
be needed. The same thing is true of 
many plant diseases. 

The effect of weather on a plant 
disease is a consequence of its action 
on the susceptible plant (the host) ; on 
the parasitic organism (the pathogen 
that causes the disease); and on the 
relation between host and parasite. 

One of the most noticeable things 
about plant diseases is that some of 
them occur wherever the host plant is 
grown and that others are restricted to 
certain parts of the host territory. 
Again, a disease that is more or less 
constantly present over a wide area 
may always be destructive in some 
locations but normally insignificant in 
others. That is because of the action of 
climate. 

Climate is the average weather of a 
locality or region. It includes the 
seasonal progress of the weather as well 
as the extremes. For a season, a year, 
or a series of years, weather may vary 
in one respect or another from the 
“normar’ for the climate of which it 
is a part, but over a long period 
climate is as definite a feature of a 
region as its soils, rivers, and forests. 
In fact, climate is largely responsible 
for the regional characteristics ex- 
pressed in the landscape. 

So it is with a plant disease. The 
total effect of weather upon it is 
summed up in its geographical dis- 



YEARBOOK OP AORICULTURE 1953 


84 

tribution. Wc cannot always be sure 
of the precise explanation, but wc 
know that most diseases flourish best 
in certain kinds of climate. Just as 
with the plants that they attack, some 
pathogens prefer cool regions and 
others are restricted to warmer areas; 
some require a great deal of moisture 
and others get along with less. 

Thus the climate of a region deter- 
mines the crops that can profitably 
be grown there and also the diseases 
to which the crops are subject. To 
put it the other way around: Given 
the presence of susceptible plants, the 
area of occurrence of a plant disease 
depends primarily on the climate. The 
outside boundaries of distribution gen- 
erally are the extremes of hot and cold 
and wet and dry that the parasite can 
endure. 

Within a range of conditions that 
permit its existence, a plant disease 
may be an insignificant or an im- 
portant factor in crop production, 
depending on how exactly the local 
or seasonal conditions fit the require- 
ments for its development and spread. 
Also, within this range, some factor 
other than weather may a.ssume the 
decisive role — susceptibility of host 
varieties, for example, or the type or 
reaction of soil, or cultural manage- 
ment. 

A few examples illustrate some ways 
in which climate is responsible for 
the distribution and importance of 
plant disease. 

Apple scab, caused by the fungus 
Venturia inaequalis, occurs almost every- 
where apples arc grown. It is absent 
or unimportant only in hot or very 
dry regions. In areas with cool, rainy 
springs, it becomes a limiting factor 
and constant attention is required to 
control it. In the Northeastern and 
North Central States, climate is es- 
pecially favorable to scab, which is by 
far the most important apple disease 
there. 

During a series of surveys to deter- 
mine the inudence of cotton seedling 
diseases and boll rots, technicians 
discovered that the occurrence and 


nonoccurrence of anthracnose caused 
by Glomerella gossypii were definitely 
separated by a line running through 
eastern Texas and Oklahoma. The 
line nearly coincided with the bounda- 
ry between below 10 and over 10 
inches of average summer rainfall. 
Anthracnose is a constant and impor- 
tant factor under high summer rain- 
fall in the eastern part of the Cotton 
Belt, but is practically nonexistent in 
the western part, which has lower 
rainfall. 

Onion plants are susceptible to in- 
fection by the soil-borne spores of the 
smut fungus, Urocystis crpulae, for only a 
very short period in the seedling stage 
before emergence.. Warm soils hasten 
the growth of the seedling and allow 
it to escape infection. In the South, 
onion seed mostly is planted in the fall 
and germinates in warm soil. Smut 
is scarcely know'n in the South, there- 
fore, although it must have been intro- 
duced time after time with onion sets. 
By contrast, the disease is important 
in old established onion-growing dis- 
tricts from Kentucky northward, and 
is still spreading in northern sections. 

Some diseases occur widely because 
the time of year at which susceptible 
plants are grown in different regions 
favors their development. 

Potato late blight, caused by the 
fungus Phytophthora injestansy which 
requires cool and moist weather, is one 
such disease. Most races of the fungus 
cannot survive high summer tempera- 
tures. The disease occurs in the South 
because potatoes are a winter and 
spring crop there and because the 
organism is reintroduced with infected 
seed tubers each season. 

Climate can constitute an effective 
barrier against the advance of a plant 
disease from one region to another. 
For instance, the Great Plains seems 
to act as a barrier against curly top of 
sugar beets. The virus that causes 
the disease affects many other kinds 
of plants, including tomato, cucurbits, 
and beans. The disease occurs in the 
intermountain region and westward 
from north to south, but not farther 



THI IFFECT OF WEATHEE ON DISEASES 


east than the western edge of the 
Great Plains. Its failure to spread 
eastward may be due to some climatic 
relation of the insect that carries the 
virus, the beet leafhopper Citculifer 
Unellus. 

Several important soil-bome organ- 
isms cannot stand low temperatures 
for long. Among them are the bac- 
terium Xanthomonas solanacearum^ which 
causes Granville wilt of tobacco and 
attacks many other kinds of plants; 
Sclerotium Toljsiiy the cause of southern 
blight; Macroph»mina phaseoliy which 
causes charcoal root rot or ashy stem 
blight; and Phymatotrichum omnivorumy 
to which the so-called Texas root rot 
of cotton is due. All attack many 
kinds of plants. All are practically 
restricted to the southern part of the 
country. The cotton root rot fungus is 
further limited to the southwestern 
region, where it is native to certain 
types of soil. The first three organisms 
have been carried by various means 
to more nortliern areas but are not 
likely to become constantly present 
there because of their temperature 
limitations. 

A soil-borne organism that prefers 
cool temperatures but can endure 
warm periods can achieve a much 
wider permanent distribution. Scle^ 
Totinia sclerotiorumy another fungus with 
a long list of hosts, occurs in nearly 
all parts of the country. In the South 
it is active during cool weather. 

A DISEASE and the pathogen that 
causes it pass through a series of 
stages: Carry-over of the pathogen 
from one season to another; primary 
(first-season) infection of the host by 
^e pathogen; growth of the pathogen 
within the tissues of the host; repro- 
duction of the pathogen and secondary 
spread to new host plants; and finally 
production of the carry-over stage of 
the pathogen again. 

At each of these stages temperature 
and moisture must be within a range 
(depending on the particular disease) 
that permits the process to continue. 
A cumulation of favorable stages 


85 

induces severe disease. Unfavorable 
conditions at any of the stages retard 
development of the dbease or may 
stop it entirely. 

Either temperature or moisture C2Ln 
be decisive in the initiation, develop- 
ment, and spread of disease. If one is 
constantly favorable, the other be- 
comes the limiting factor. If both 
temperature and moisture fluctuate, 
both must be favorable at critical 
times. If both are constantly favorable, 
the disease becomes serious. 

The amount of inoculum (infective 
stage of the pathogen) that wall be 
available to start new infections at 
the beginning of a season depends on 
the extent of infection at the end of 
the preceding season and on how well 
the pathogen can overwinter. Abun- 
dant overwintering and favorable 
conditions for infection in the early 
part of the season lead to heavy 
primary infection; if conditions remain 
favorable, the advantage of the early 
start is maintained throughout the 
growing period of the cijpp. 

Some pathogens overwinter in the 
tissues of their hosts — for example, the 
potato late blight fungus in infected 
tubers, the peach bacterial spot organ- 
ism, Xanthomonas pruniy in cankers, and 
the numerous seed-bome pathogens in 
seeds. Soil fungi and bacteria may 
overwinter on host debris. Many 
fungi, for instance many of the cereal 
smuts, produce a particular kind of 
spore ^at is able to survive the 
vrinter temperatures. Some of the 
spores will not germinate unless they 
have been subjected to low tempera- 
tures. Sometimes alternating warm 
and cool or wet and dry periods arc 
required for spore germination or to 
complete the development of primary 
inoculum. 

Some fungi, in addition to or 
instead of spores, develop a different 
kind of special organ to carry them 
over unfavorable periods of various 
kinds; these are sderotia. 

A pathogen may overwinter in dif- 
ferent forms in different regions. In the 
United Sutes the apple scab fungus 



86 


YEARBOOK OF AGRICULTURE 1953 


overwinters and completes its develop- 
ment in fallen leaves, and spring infec- 
tion Ls started by spores of the perithc- 
cial, or perfect, stage — the ascosporcs. 
In England the perithecial stage is 
rare, and the vegetative stage (myce- 
lium) of the fungus is carried through 
the winter in infected twigs and buds, 
producing the ordinary summer spores, 
or conidia, when growth begins again. 

Another example is the cereal stem 
rust fungus, Puccinia gramims. The sum- 
mer spores, or uredospores, which are 
responsible for continuous spread of 
summer infection, can survive the win- 
ter in southern Texas and sometimes 
somewhat farther north. In northern 
areas, how'ever, the winter spores, or 
teliospores, are necessary for w'intcr 
survival. This fungus is a rather special 
case, since its winter spores infect not 
grains but barberry, on which another 
kind of spore is produced that will in- 
fect grains and grasses in the early part 
of the season. 

During periods of high temperatures 
or dry weather? many disease-produc- 
ing pathogens cease progress and re- 
main quiescent in established infections 
or exist as saprophytes in ho.st debris. 
The cotton anthracnose fungus is an ex- 
ample. Between seedling infection and 
boll infection, it resumes active growth 
and spread wdth every rainy period. 

The microclimate furnished by the 
host plant itself undoubtedly aids sur- 
vival of many pathogens in hot, dry 
weather. Within the plant, air is hu- 
mid, shaded, and comparatively cool, 
quite different from the outside atmos- 
phere. 

The period of primary infection is a 
critical stage. No matter how abun- 
dant overwintering may have been, if 
weather then is not favorable the path- 
ogen cannot infect its host. Take apple 
scab as an example. Primary infection 
by the pathogen requires that asco- 
sporcs produced in the fallen leaves 
mature at the same time that unfolding 
leaves and buds on apple trees arc sus- 
ceptible to infection, which is only dur- 
ing the short period of their active 
growth. Further, there must be a rainy 


period lasting long enough to keep the 
new host surfaces constantly wet for 
some hours, at a high enough tempera- 
ture. Ordinarily in the Northeastern 
and North Central States the time of 
ascospore production, progress of host 
development, and spring weather all 
favor the disease. In a dry spring, how- 
ever, dissemination and germination of 
ascospores are inhibited. In some ex- 
ceptional seasons, also, most of the as- 
cosporcs may “shoot” before trees are 
ready for infection; that is, before foli- 
age is produced. Wh®n primary infec- 
tion is reduced or delayed, subsequent 
spread is likely to be reduced also, un- 
less a very favorable period occurs 
later in the season. 

Later stages in disease development 
are similarly affected by weather, al- 
though some diseases (after infection 
by the pathogen is established) are rel- 
atively indifferent to subsequent condi- 
tions. Other diseases, how ever, respond 
quickly to any change in temperature 
or moisture. Rather uniform environ- 
ment throughout their course favors 
some pathogens. Others grow’ well un- 
der one set of conditioas but may 
require the stimulus of a sudden de- 
cided change to induce sporulation, 
while still a different combination may 
be necessary for spore germination and 
for infection. 

Often the connection between dis- 
ease and weather is obvious — the at- 
tacks of potato late blight following a 
cool rainy period, for instance, or the 
dependence of the apple scab on rain. 
With other di.seases, however, the* crit- 
ical period has been passed long before 
the attack becomes evident. For ex- 
ample, the temperature of the preced- 
ing winter is the decisive factor in the 
occurrence of bacterial wilt of sweet 
corn (caused by Bacterium stewartii): on 
the other hand, the severity of wheat 
leaf rust depends on temperature and 
moisture during late winter and early 
spring. The weather at the time that a 
plant disease is most conspicuous is not 
necessarily a reliable clue to the condi- 
tions that encourage disease develop- 
ment. Moreover, we cannot generalize 



THC B>FfiCT OF WfATHBR ON DISEASES 


from one disease to another. Each one 
must be studied separately. 

It is hard to determine the exact 
relationship of disease to weather be- 
cause of the variability of the weather, 
the lag between the critical period for 
infecuon of the host by the pathogen 
and subsequent development of disease 
syHiptoms, and the fact that best con- 
ditions for production of a disease may 
not coincide with best conditions for 
growth of its causal organism. This 
last apparently paradoxical situation 
exists because disease is the result of 
interaction between host and patho- 
gen. Conditions that are most favor- 
able for growth of the pathogen in 
artificial culture may allow the host to 
withstand or escape attack. In such 
cases disease can occur only with some 
other combination at which the host 
will be vulnerable and the pathogen 
will still be active. Therefore, the 
disease-producing requirements of the 
pathogen must be a compromise be- 
tween what is most desirable for its 
pwn processes and what is actually 
possible in its relations with its host. 

Change in the intensity of one 
weather element brings about changes 
in the whole disease relationship. A 
change in temperature may make 
existing moisture conditions more or 
less favorable to attack by the jjatho- 
gen, or it may increase or decrease 
the vulnerability of the host. Con- 
versely, change in moisture supply 
may require a corresponding change 
in temperature if the disease is to pro- 
gress unchecked. The struggle be- 
tween host and pathogen is sometimes 
so delicately balanced that a very 
small alteration in only one condition 
is enough to assure victory to one side 
or the other. 

If both a host and its parasite in 
their separate existence are favored by 
the same range of conditions, and if 
this range does not also enable the 
host to resist attack, then the disease 
caused by the pathogen is apt to be- 
come a limiting factor in the culture 
of the host, in any place where these 
conditions are usual. So we have the 


87 

seemingly contradictory fact that best 
yields will be obtained under circum- 
stances known not to be particularly fa- 
vorable to the host. On the other hand, 
if best conditions for the host arc differ- 
ent from those most favoring growth of 
the pathogen by itself, or if conditions 
suiting both make the host more re- 
sistant, the disease will result only 
when the pathogen gains the advan- 
tage over its host — for example, 
when the growth rate of the host is 
slowed down so that it remains in a 
susceptible state for a long enough 
time to allow infection by the patho- 
gen, or when injury from suffocation 
by waterlogged soil permits root- 
rotting fungi to invade the rootlets, 
or when the chemical or mechanical 
constitution of host tissues is affected 
in such a way as to favor the pathogen. 

A good example of the differential 
effect of temperature on host and 
pathogen and on the development of 
disease is given by the fungus Gib- 
berella zeae^ which is secd-bornc and 
also overwinters in debris left in the 
soil. It is widely distributed through- 
out our more humid corn and wheat 
sections and causes seedling blight 
and other diseases of corn and small 
grains. In artificial culture it grows 
best at 75.2® to 8.0.6° F.; the mini- 
mum and maximum are 37.4° and 
89.6°. Wheat likes fairly low soil 
temperatures. Corn prefers warmer 
soils. If other growing conditions are 
favorable, little or no seedling blight 
develops on wheat at 53.6°. As the 
temperature increases, so docs the 
amount and severity of the disease, up 
to as high as 80.6°. With corn, no 
seedling blight results above 73.2°, 
and the most favorable range for the 
disease is from 46.4° to 68°. The 
explanation is that with each host 
grown under its most favorable con- 
ditions, the cell walls of the host tissue 
are more resistant and the reserve food 
supply is less attractive to the patho- 
gen. With wheat, higher temperatures 
^ter composition of cell walls and 
food supply to make this host moi*e 
vulnerable to infection and the more 



88 


YEARBOOK O F A G R1 C U L T U R E 1R53 


suited to the pathogen’s food require- 
ments. The reverse is true with corn; 
high temperatures make it more re- 
sistant. Evidently, in both instances, 
the temperature effect is due to action 
on the growth processes of the host. 
The practical result Is that seedling 
blight due to this fungus attacks wheat 
mostly in the southern part and corn 
in the northern part of the range of 
the host. 

Cabbage yellows is caused by a 
soil-borne fungus, Fusarium oxysporum 
f. conglutinans. The fungus grows best 
in warm temperatures. The disease is 
associated with warm weather. Here 
it is the fungus that is affected by 
temperature: Yellows is rare in south- 
ern cabbage-growing areas because 
cabbage is a winter crop there and 
grows in cool soils. 

Some diseases caused by soil- borne 
organisms arc more closely dependent 
on moisture than on temperature. One 
such disease is the avocado root rot, 
caused by Phytopkthora cinnamomi^ in 
California. Plantings on overirrigated 
or flooded locations are severely 
affected. Exce.ss moisture prevents 
soil aeration. The rootlets are injured 
by lack of oxygen, and the fungus 
gains entry to the roots through the 
injured portions. The disease does not 
occur on waterlogged soils that do 
not contain the fungus, although the 
rootlets are as badly injured. 

For pathogens that affect the above- 
ground parts of plants, environment 
is much more complex and variable 
than for the soil-borne organisms. 
Alternation of night and day is accom- 
panied by changes of temperature as 
well as light. Cool air can hold less 
water vapor than warm air. If the 
difference between day and night 
temperatures is great enough in rela- 
tion to daytime humidity, dew forms 
on the plants. Quiet air preserves 
vertical and horizontal differences in 
temperature and moisture. Air move- 
ment mixes cool and warm, wet and 
dry air. It aids evaporation by pre- 
venting accumulation of moisture- 
saturated air and promotes drying of 


moist surfaces. Dew will not form on 
windy nights. Moving air carries in- 
fective stages of pathogens from place 
to place. Rain, dew, and fog wet the 
surfaces of plants and supply moisture 
for germination of fungus spores and 
multiplication of bacteria. Inoculum 
on the soil surface may be borne to 
low stems or leaves or low-hanging 
fruit with splashing raindrops. Drip- 
ping moisture from fog or rain can 
iarry infection from tree tops to low- 
est limbs. Wind-driven rain may 
transport spores and bacteria for con- 
siderable distances. Cloudiness and 
sunshine affect temperature, evapora- 
tion, humidity, air movement. 

The constant trouble that plant dis- 
eases cause might lead us to think 
every bit of infective material succeeds 
in producing infection. That is not so. 
Most of the countless fungus spores 
every season fail to survive all the haz- 
ards against them and reach the right 
part of the right kind of plant during 
the life period of the spore. If it falls on 
anything else, a S[>ore is so much dust. 
If it does Anally fall on the surface of its 
host, it must be on a part and during a 
stage vulnerable to infection, without 
a coating of fungicide that w^ould kill 
the spore, and with sufficient moisture 
or humidity and the proper tempera- 
ture lasting for a long enough period to 
allow the spore to germinate and pene- 
trate the host tissue. 

A pathogen carried by an insect, as 
are most viruses and many fungi and 
bacteria, has an advantage over air- 
borne organisms. The insects take the 
pathogen more or less straight to the 
proper host, and in most cases inocu- 
late it directly into the host tissues. 
Such pathogens and the diseases they 
cause are nevertheless just as subject to 
the effect of weather > The vectors, in- 
deed, add a third organism to be in- 
cluded in the disease-weather relation. 
Weather affects survival, increase, and 
activity of the insects, as well as dircc- 
lionj distance, and intensity of migra- 
tion and flight. Overwintering of the 
insect vector is as important as survival 
of the pathogen itself. Some of the 



THE EFFECT OF WEATMEE ON DISEASES 


pathogens even overwinter in the 
vector. 

Some of the most dreaded diseases 
of crops are of moderate importance 
or are scarce or even absent a good part 
of the time. But they can attack with 
great suddenness and destructiveness 
in certain seasons, or perhaps during 
several consecutive years. Among them 
are wheat stem rust and potato late 
blight, which are probably the most 
famous of plant diseases because of the 
importance of the hasts almost every- 
where and because of the extraordi- 
nary severity of epidemic outbreaks of 
either disease. 

The incidence of stem rust in our 
central Wheat Belt goes sharply up and 
down from high peaks in favorable 
seasons to low troughs in years when 
the disease is negligible or absent. 
Widespread outbreaks are less frequent 
than formerly because of the use of 
varieties resistant to the predominant 
physiologic races of the organism and 
because of the eradication of barberry, 
which was an important source of carJy- 
season infection in the northern part of 
the region. Nevertheless, every once in 
a while the chain of circumstances still 
favors the disease and an epidemic 
results. 

The essential features are: Rapid 
build-up of a physiologic race of stem 
rust, to which the wheat varieties 
predominantly grown are susceptible; 
mild winter weather, which allows 
abundant uredospore overwintering, 
perhaps as far north as Oklahoma; 
favorable damp and not too warm 
weather for early infection in the 
southern part of the region and con- 
tinuing favorable weather along the 
way, as spores are borne northward by 
the wind, to produce infection step by 
step until the northern whcatfields arc 
reached. Finally, hot, dry weather is 
necessary during the period when the 
wheat kernels are forming on the rusted 
plants to produce maximum damage. 
Such a favorable combination resulted 
in the 1935 epidemic, which cost the 
country almost a quarter of the crop 


89 

for that year. In Minnesota and 
North Dakota the loss was almost 60 
percent. 

A graph of the occurrence of potato 
late blight would show as pronounced 
ups and downs as for stem rust. In the 
East, primary outbreaks follow two 
weeks of rainy or foggy, cool weather. 
The earlier in the season primary in- 
fection occurs, the more severe the 
outbreak will be, if moisture and 
temperature are at the required levels. 
In extremely favorable seasons, late 
blight spreads with explosive rapidity. 
The very conditions that favor the 
disease make it exceedingly difficult 
to keep plants protected by fungicides. 

Naturally, the same conditions, when 
they exist, would induce severe attack 
in the Central States also. A more 
frequent favorable combination in 
that region, however, is alternation of 
high daytime temperatures with low 
night temperatures and high relative 
humidity during the day. The marked 
temperature fall at night stimulates 
sporulation and forces excess water 
vapor out of the air as dew, which 
supplies moisture for germination and 
infection. The microclimate furnished 
by the plant may be an important 
factor in the region. The result is 
serious, continued spread under con- 
ditions that, without analysis, would 
appear unfavorable. 

In any area, really dry weather, or 
constantly high temperatures, and 
especially a dry, hot season will pre- 
vent the occurrence of late blight or 
check its spread. Severe epidemics 
result from an early start with abun- 
dant inoculum and continuous favor- 
able temperature and moisture. 

Late blight has a habit of appearing 
suddenly and destructively in regions 
where it had not previously been 
considered a factor. The epidemic in 
the southern potato crop in the winter 
and spring of 1943-1944 is a good 
illustration. Contributing factors in 
that outbreak were: The especially 
abundant supply of inoculum that 
resulted from wartime relaxation of 
seed requirements; exceptionally wet 



YEARiOOK OF AGRICULTURE 1f53 


90 

weather; favorable temperatures; lack 
of experience with the disease; and 
difficulty of control, the result of the 
early heavy attack, weather that pre- 
vented efficient application, and scar- 
city of control materials. The total 
result was tlie most severe and wide- 
spread epidemic ever known in the 
South. Losses were heavy in some 
States where the disea.se had not been 
seen for 30 years or more. 

Late blight is also a disease of tomato. 
Since 1940 or so several severe out- 
breaks have caused heavy losses to 
tomato growers. In 1946 tlie disease 
caused an estimated loss of 40 million 
dollars and brought sharply to at- 
tention our need to use all the knowl- 
edge that we already possess about 
weather and disease and to learn a 
great deal more, if we are not to be at 
the mercy of late blight and other 
epidemic disea.ses. 

Tomato late blight had been severe 
in various limited areas at different 
times before 1946. In 1946, however, 
all factors worked together and the 
result was unprecedented severity and 
heavy loss along the Atlantic coast 
from Florida to New England. The 
outbreak extended as far west as 
Minnesota in the North and Texas in 
the South. Again, late blight demon- 
strated its capacity to move rapidly 
and destructively into sections previ- 
ously free of it. 

The disease was severe on the 
winter and spring crop in the South. 
Plants grown in Georgia and other 
Southern Slates for early planting 
in northern fields became generally 
infected. Weather at the time of 
planting in the North favored the 
spread of infection from this very 
abundant and active source of inoc- 
ulum and continued to favor the 
disea.se for most of the season. As 
usual with such sudden, widespread 
outbreaks, preparations for control — 
equipment, materials, and experi- 
ence — were inadequate. 

Late blight belongs to a group of 
diseases, the downy mildews, that are 
especially sensitive to weather. An- 


other member of this group is the blue 
mold or downy mildew of tobacco, the 
causal fungus of which is Peronospora 
tabacina. Thirty years of records have 
shown that in southern tobacco-grow- 
ing sections the incidence of blue mold 
is greatly influenced by temperature 
in January. Above-average temper- 
atures are followed by early appear- 
ance; with low temperatures, infection 
is slow to develop. Subsequent condi- 
tions also affect severity; the most 
widespread, severe attacks occur in 
years when high January tempera- 
tures permit early infection, and tem- 
perature and moisture conditions after- 
ward favor continued spread. 

Experience with sweet corn bac- 
terial wilt shows how we can make 
use of observed facts and records about 
w^eather and occurrence even though 
at first we do not know the explana- 
tion for the behavior of the disease. 
Bacterial w'ilt is constantly pre.sent 
on su.sceptible varieties in the South, 
but as a rule it occurs only occasion- 
ally farther north. During the early 
1930^8 general and severe attacks 
made it the major disca.se of the crop 
in north central and northeastern 
sections of the country. 

Study of a long series of observa- 
tions recorded in Connecticut, to- 
gether with the weather records, indi- 
cated that the disease followed warm 
winters and w^as .scarce or absent after 
average or cold winters. The seasons 
during which bacterial wilt had be- 
come increasingly severe were pre- 
ceded by unusually warm winters. 

It was found that .summation of the 
average temperatures for December, 
January, and February for any given 
place would show very accurately 
whether or not the disease could be 
expected during the following season 
and how much damage it would cause. 
If the sum, which is called the “winter 
temperature index,’’ is 100 or above, 
the disease will be destructive; with 
lesser sums, incidence is correspond- 
ingly less severe. 

Since the correlation between winter 
temperature and disea.se incidence was 



THE EFFECT OF WEATHER ON DISEASES 


established, it has been found that it 
depends on winter survival of the 
insect vector, the flea beetle Chaetoc- 
nema pulicularia. This fact explains cer- 
tain small discrepancies between ob- 
served occurrence and the original 
statement of the correlation. In some 
seasons following a very warm winter, 
incidence of bacterial wilt was less 
than was to be expected. These sea- 
sons usually were preceded by one or 
several during which the disease was 
minor. More than one favorable sea- 
son is required for both the vector and 
the Vjactcrium to build up sufficiently 
to result in maximum disease inci- 
dence. 

The correlation is so regular that it 
is used as a guide to planting. Re- 
sistant sorts are used when severe oc- 
currence is indicated. 

Several points need to be noticed 
about this disease-weather relation- 
ship. First, it required study of both 
disease occurrence and weather records 
to establish it. Second, it was useful 
even before the explanation w^as 
known. Third, it involves a third 
organism, the vector. Fourth, it gives 
a chance to take measures against the 
disease even before seed is planted. 

In many ways, the fact that diseases 
are so dependeni on weather is a great 
advantage to us in our fight against 
them, as we .shall see now. 

One cannot study a parasitic plant 
disease without taking into account 
the influence of temperature and mois- 
ture on the pathogen, on the reaction 
of the host, and on consequent disease 
development. Obviously, a connec- 
tion so regular must have great prac- 
tical significance. 

One way in which we can make the 
disease-w'eather relationship work for 
us is in the management of various 
agricultural operations to take advan- 
tage of temperature and moisture con- 
ditions unfavorable to disease develop- 
ment. Thus, the way in which irriga- 
tion, surface or overhead, is used can 
cither favor or discourage attack by 
pathogens. Many diseases attacking 


91 

through the seedling can be controlled 
by so timing planting that the seed 
will start growth under conditions 
inhibiting infection by the pathogen. 
Temperature and humidity in green- 
houses and storage houses can be 
maintained at levels that will prevent 
or control disease. Arid and semiarid 
locations are ideal for the production 
of seed free from seed -borne pathogens, 
since the requisite moi.sture for spore 
germination and bacterial spread is 
lacking in their climates. 

Chemical control measures can be 
used wdth precision w'hen climatic 
relationships are taken into account. 
For example, seed treatment for cotton 
seedling disease gave varying results 
before surveys show'cd that seed -borne 
anthracnosc was by far the most im- 
portant cause in the humid Southeast, 
and that soil-borne organisms pre- 
dominated in drier regions westward. 
When this different disease distribution 
was knowm, seed treatment could be 
applied specifically for each type of 
cause and was much more effective. 

We can avoid the inevitable failure 
that would result from planting a crop 
in a region where it would be subject 
to a disease hazard because of weather. 
We can be watchful also to prevent the 
introduction of new^ diseases that would 
create new hazards in favorable cli- 
mates. 

It does not take very much imagi- 
nation to realize that many of the 
diseases we have already discussed 
would constitute hazards in particu- 
larly favorable regions, if the control 
measures were not known. In fact, 
after a series of seasons when WTather 
especially suits disease attack and 
makes control efforts arduous, expen- 
sive, and inefficient, growling of the 
host may have to be abandoned or 
greatly reduced for a while. 

If we can tell when an outbreak is 
likely to happen, we can prepare for it 
and reduce losses. In particular, we 
can overcome the difficulty in the use 
of expensive chemical control measures 
arising from the fact that routine appli- 
cation is wasted in years when the 



g2 YBAKBOOK OP ACKICULTUBB f953 


disease is absent, but, on the other 
hand, when it does attack protection 
must be prompt and continuous to do 
any good. With such a choice farmers 
are apt to take a chance and often will 
sustain severe losses. Forecasting en- 
ables sound judgment instead of waste- 
ful guessing on the need for control 
measures. 

Prediction docs not always help in 
control but does enable farmers to 
reduce their losses in other ways. For 
instance, there is no practicable short- 
time control measure available for 
wheat leaf rust, but forecasts issued 
early in the season allow farmers to 
plow up their wheat and plant some 
other crop or to pasture their fields if 
a serious outbreak is indicated. Thus 
they can recover at least part of their 
season’s investment. 

Predictions can never be entirely 
right. This is just as true of plant 
disease forecasting as of weather or 
political polls or anything else. The 
average correctness for the limited 
number of plant diseases that arc being 
forecast so far is more than 8o percent. 
The possible accuracy depends on 
how complicated the critical periods 
arc and on how far in advance they 
operate. For instance, if winter tem- 
perature by itself determines the 
amount of the disease for the following 
season, a prediction is a simple matter 
of calculation and can be made with 
practically complete certainty that it 
will be right. In contrast, if the patho- 
gen can cause disease at any time that 
its temperature or moisture demands 
or both are met, both disease and 
weather must be watched throughout 
the season, and frequently revised 
short-term forecasts are necessary. 

Forecasting plant disease occurrence 
on the basis of known weather require- 
ments is well established. In this 
country apple scab spray warning 
services have been routine for almost 
30 years. Some other diseases for which 
forecasting is regularly a part of the 
control program in one region or an- 
other include bacterial wilt of sweet 
corn, wheat leaf rust, late blight, 


tobacco blue mold, cucurbit downy 
mildew {Pseudoperonospora cubensts), and 
lima bean downy mildew {Phytoph^ 
tkora phaseoli). The weather relations 
that make it possible to predict for 
some of these diseases have already 
been indicated. 

The 1946 tomato late blight out- 
break proved beyond doubt that 
diseases that spread so rapidly, and 
attack so destructively, require more 
than local attention to cope with them 
adequately. As a result, the Crop 
Plant Disease Forecasting Project of 
the Plant Disease Survey, Bureau of 
Plant Industry, Soils, and Agricultural 
Engineering, was organized. The proj- 
ect is a concrete expression of the 
importance of the weather-disease 
relation. It was set up especially to 
keep watch over circumstances that 
might lead to outbreaks of plant 
diseases, to use all available informa- 
tion about conditions favoring diseases 
as an immediate basis for making 
forecasts, and to study the require- 
ments for development and spread so 
that predictions can be improved in 
accuracy and duration. 

The chart on the next page shows 
how the warning service of the project 
operates. A pathologist in each State 
works with it and performs the same 
task for his own State that the project 
does for regions. Because information is 
obtained from such a wide area, ample 
time is given for local warning and 
preparation if a disease outbreak seems 
likely. An indispensable feature of the 
project, noted in the chart, is the 
cooperation of fungicide and equip- 
ment manufacturers, which assures 
availability of chemicals and control 
equipment wherever and whenever 
they arc needed. The role of the 
Weather Bureau, in a program that 
has weather as one of its components, 
is obvious. 

Late blight, tobacco blue mold, and 
cucurbit downy mildew, up to now, 
have been the chief diseases consid- 
ered by the warning service. Regional 
forecasting for pouto late blight has 
been tried in the North Central States, 



THE EFFECT OF WEATHEB ON DISEASES 


93 



with good results. The forecasts are 
based on temperature and humidity 
recorded by hygrothermogi aphs placed 
in potato fields in various parts of the 
region, on the weather forecasts, and 
on observations on the prevalence of 
the disease in selected locations. This 
regional forecasting on the basis of in- 
strumental recording, much the same 
as for weather, is a new trend and 
shows promise of further development. 

We can make useful predictions 
without knowing all the reasons for 
the observed reaction of disease to 
weather. Neither do we need to know 
at the time of predicting exactly what 
the weather will be. We can say that 
on the basis of what we know now, ij 
the weather is this, then the disease will 
be that, and even the conditional fore- 
cast will be helpful. Obviously, how- 
ever, the more we know about the 
operation and timing of the factors 
favoring or inhibiting the develop- 
ment and spread of disease, and the 
longer ahead of time we can be sure 
of the weather, the more accurate our 
predictions will be and the earlier we 
can make them. Improvement and 
extension of the long-range w^eather 


forecasts is the answer to the latter 
need. The research program of the 
forecasting project is designed to add 
to our knowledge on the connection 
betw'cen environment and disease. 
Detailed and continuous observation 
of weather and microclimate is as 
much a part of its investigation as are 
the purely disease factors. By means 
of w’ealher-recording instruments, the 
weather can be correlated directly 
with the development of the disease in 
experimental fields. The regional late 
blight forecasts in the Central States 
are a result of this sort of study. 

Paul R. Miller, a pathologist in 
charge of the Plant Disease , Survey, 
Bureau of Plant Industry, Soils, and 
Agricultural Engineering, has spent j6 
years developing survey techniques and 
conducting field studies tf diseases that 
occur on major economic crops, including 
peanuts, tobacco, cotton and truck crops. 
Dr. Miller directs a plant disease fore- 
casting service in cooperation with the 
agricultural experiment station pathologists 
of the 48 States, the United States Weathei 
Bureau., and the National Fungicide and 
Farm Equipment Association. 









94 


YIAttOOK OP AORICUlTURi 1«SR 


Environmental, 
Non parasitic 
Injuries 

J. E. McMmtriy^ Jr» 

Bad weather, air pollution, growth 
regulators, and the deficiencies or ex- 
cesses of minerals in the soil can cause 
a group of diseases of plants that we 
classify as environmental and non- 
parasitic. 

They are related closely to diseases 
brought about by parasitic organisms. 
Often we can regulate plant growth so 
as to control them and thereby con- 
trol somewhat the diseases attributed 
to parasites. Symptoms associated with 
nonparasitic diseases frequently arc 
confused with those caused by fungi, 
bacteria, and viruses. Often the injury 
from nonparasitic disturbances per- 
mits fungi, bacteria, or viruses to enter 
and damage the plant. 

The severity and type of injury vary 
with the plant, its stage of maturity 
when the disturbance occurs, and the 
part of the plant involved. 

Lightning, hail, wind-blown rain, 
drowning, frost, and drought arc 
among the elements of weather that 
may harm plants. 

Lightning may tear large trees apart 
or it may only injure a few limbs. It 
may kill the stem tissue of annual 
plants, such as tobacco, so that tlie 
leaves get a shrunken, dark midrib. 
Damage to plants by lightning is local 
and usually not extensive. 

Hail may cause only small holes in a 
few leaves or complete defoliation and 
destruction of plants. A striking in- 
stance of hail damage is the destruc- 


tion of an entire field of shade-grown 
tobacco and the shade cloth over it. 
Usually hail does damage in limited 
areas only. 

Heavy rains may break young, ten- 
der leaves or puncture holes in them. 
Wind-blown rain also causes water 
soaking of the intercellular spaces of 
the leaves. Sometimes, if micro-organ- 
isms are present, the damage may be 
severe. Plants blow over; leaves and 
grain in contact with the soil may rot; 
it might be impossible to use machin- 
ery to harvest the crop. 

Most crop plants grow well on rela- 
tively well-drained soil that may be 
subject to leaching or temporary ■flood- 
ing, but most plants will not survive 
persistent flooding, which drowns and 
destroys the root system. If a part of 
the root system is damaged, growth is 
reduced and micro-organisms may 
invade the tissues. Temporary wilting 
is often evident. In sandy soils the 
rapid loss of water by percolation — 
leaching — cau.ses the loss of soluble 
plant foods, particularly nitrogen and 
possibly magnesium. 

Not uncommonly are plants injured 
on days of high temperature and 
bright sunshine. Sunscald is the per- 
manent wilting of young leaves. An- 
other type of injury results in a drying 
of the lower or older leaves. vSuch con- 
ditions are more common with tem- 
porary shortages of water. Corn, for 
example, first shows rolling of leaves; 
if the drought continues it may suffer 
to the degree that the upper part of 
the plant, including the staminate 
inflore.scence, dries up and fertiliza- 
tion cannot take place. In extreme 
droughts trees and other plants may 
die. 

In cold weather growth may be 
delayed so tliat parasites develop. 
Losses from late spring and early fall 
freezes arc an ever-present threat in 
most of the Temperate Zone. Small 
grains, com, and other crops often fail 
to reach proper maturity before being 
killed by freezes in some seasons in 
northern latitudes. Following frost 
and freezing injury, plants may suffer 



ENVIRONMf NTAL. NONPARASITIC INJURIES 


death of twigs and branches, splitting 
of trunks, and the loss of fruit crops 
when flowers are killed. 

Factories may release concentra- 
tions of gases that are toxic to plant 
growth. Sulfur dioxide is an example. 
In many places the vegetation around 
industrial establishments — such as fac- 
tories that make sulfuric acid or 
smellers of sulfide ores - has been 
almost entirely wiped out. Smoke from 
coal sometimes contains sulfur dioxide 
in amounts that may injure plants if it 
is not dissipated Ijy wind. 

Fluorine, as hydrofluoric acid gas, is 
injurious to plants near chemical 
works that release it into the air. The 
injury often appears only as marginal 
lesions or necrosis, but sometimes the 
entire leaf dies prematurely. Low con- 
centrations of fluorine often cause 
leaves to turn yellow. 

Smog i.s a still, heavy mixture of fog 
and various contaminant.s, such as 
sulfur oxides, ammonia, fluorides, fil- 
terable oils, gaseous hydrocarbons, 
oxides of nitrogen, and hydrogen sul- 
fide. It is not known for certain which 
one of those gases is the culprit or 
whether two or more act simultane- 
ously to injure plants. In the south 
coastal are.i of California, for example, 
Romaine lettuce, endive, and spinach 
sufler extreme injury from smog; beet, 
celery, oats, S\vi.ss chard, and alfalfa 
suffer moderate injury. Barley, onion, 
parsley, radish, tomato, turnip, and 
rhubarb suffer slight injury. Cabbage, 
cantaloup, carrot, cauliflower, cucum- 
ber, pumpkin, squash, and Ijroccoli 
suffer no injury. Bleaching and .scorch- 
ing sometimes are evident. 

Insecticides may injure plants. Ar- 
scnicals used improj>crIy may cau.se 
shedding of leaves. The effect of 
arscnicals, particularly lead arsenate, 
may be cumulative and in time may 
kill fruit trce.s. Calcium arsenate, as 
used on cotton, may temporarily steri- 
lize the soil. Oil sprays may damage 
fruit trees. Parathion and some of the 
other newer synthetic insecticides may 


95 

cause the russeting of some varieties of 
apples. Benzene hexacliloride may 
cause the formation of strap-leaf and 
off-flavors, particularly in potatoes. 

All parts of plants, notably orchard 
trees, might be injured by bordeaux 
sprays and dusts. The leaves may show 
burning, shot holing, spotting, dis- 
coloration, and defoliation. The blos- 
soms may be injured so that no fruit is 
set. The fruit may show spotting, 
ru.s.seling, malformation, cracking, and 
shedding. 'Lhc twigs may have injuries 
of various kinds, or the entire tree may 
die if damage is unusually severe. 
Lime-siilfur also might cause lesions 
on foliage or fruit and premature fruit 
drop. The rno.st common injury is a 
dull-brown spotting of the leaves or 
burning of margins and tips. 

Many injuries have followed the 
extensive use of the growth-regulator 
herbicides, especially 2,4-D. Minute 
amounts of them are enough to pro- 
duce ill effects on plants — even the 
tiny residue in a sprayer that has not 
been cleaned thoroughly with plehty 
of hot W'ater and ammonia. Injury also 
may occur from the drift of herbicide 
spray wheel the w'ind is blowing. Leaf 
malformations occur in sensitive spe- 
cies around factories that prepare 
2,4 -D. The injury may be only a slight 
rat-tail type of grow'lh of the leaves or 
the death of trees. 

A DEFICIENCY ol any one of the 
chemical elements necessary for plant 
growth may reduce total growth of 
plants. To distinguish the efl'ect of one 
clement from that of another, one must 
examine ckxscly the affected plant. For 
example, it is not enough to say that 
a leaf Ls chlorotic; a detailed descrip- 
tion of the chlorotic pattern is neces- 
saiY and the age of tiie leaf must be 
known. A shortage of any one of 
the element.s - boron, calcium, copper, 
iron, magnesium, manganese, molyb- 
denum, nitrogen, phosphorus, potas- 
sium, sulfur, and zinc — may produce 
malnutrilional di.seases of plants. 

A sliortagc of boron in the soil results 
in poor grow th of tops and roots. Top 



YEAtlOOK OF AOIICUITUIE l«S3 


96 

sickness in tobacco, heart and dry rot 
of sugar beets, internal cork of ap- 
ples, internal browning of cauliflower 
(which first occurs as small, concen- 
tric, water-soaked areas in the stem 
and central branches of the cuixl), and 
cracked stem of celery are boron-defi- 
ciency diseases. Poor growth, yellowing 
of the terminal growth, and death of 
the terminal buds are typical symp- 
tom's in most plants. The affected ter- 
minal growth becomes brittle, breaks 
easily, and shows discoloration of 
vascular tissues. 

A deficiency of calcium shows up 
finst near tKc growing point on the 
young leaves. The growing point dies 
and the young leaves often are severely 
distorted and show a hooked tip. When 
later growth takes place, the margins 
arc irregular because of the failure in 
early development. The leaf petioles 
of many plants collapse when the 
growing points die. The floral parts, 
ncluding corolla and calyx, may show 
bnormalities. Shedding and little or 
o seed set may follow. Tobacco, to- 
mato, and potato plants show distinc- 
tive deficiency efi'ecls. Tomatoes show 
pronounced dicback of .stems, leaves, 
and fruiting branches and blo.ssom-end 
rot of fruits. Potatoes form few luljers, 
show bushy vines, and develop leaf- 
lets near the shoot tips that are small, 
chlorotic, and roll inward toward the 
mid veins. Beans, peas, clovers, and 
other legumes have pale-green leaves 
with necrotic margins. The stems may 
collapse near growing points, petioles, 
and pedicels. Pods and seeds are few 
and poorly developed. The growing 
points of sugar beet, carrot, parsnip, 
and other root crops may die. The tree 
fruits undergo death of the terminal 
shoots. The tip leaves have a scorched 
and ragged appearance, and the mar- 
gins roll inward. Calcium deficiency 
in most plants results in breakdown of 
the mcristematic tissues in stem, root, 
or any part of the plant where the 
deficiency occurs. Extreme shortages 
of calcium commonly mean the early 
death of the plant. 

Years ago growers learned that the 


dieback, or exanthema, of citrus trees 
in Florida could be corrected by the 
use of copper compounds, although 
the ailment was not recognized at 
first as due to a copper deficiency. 

Apple, pear, and plum trees show 
much the same symptoms when they 
lack copper. Tobacco plants deficient 
in copper suffer a breakdown of older 
leaves and wilting of younger leaves. 
When the shortage of copper operates 
after flowering, the seed head cannot 
.stand erect and the seed stalk bends 
to one side. The cereals show much 
the .same symptoms — withering of tips 
of the younger leaves, wilting of the 
foliage, dwarfing, distorting of seed 
heads, and less formation of grain. 
The low^er leaves and tillers on such 
plants tend to remain green. Copper 
is essential for normal color and 
growth of lettuce and onions, particu- 
larly wiien they are grown on peat 
soils. 

The first deficiency disease of plants 
to be recognized was the one caused 
by too little iron. It was first reported 
in France, and the remedy was iron 
salts. Yellowing of the young growth 
is the first sign of the disease. Some 
necrosis may occur. In extreme cases 
the young leaves may become almost 
white. In milder cases there i® a mot- 
tled patteni; the primary and second- 
ary veins tend to retain the green 
color. Sometimes there is drying or 
scorching of leaf tips and margins. In 
extreme cases dicback of twigs may 
extend to large branches of trees. 
Fruit and shade trees are often more 
commonly affected than field or vege- 
table crops. The typical chlorosis due 
to iron deficiency often occurs on soils 
of high lime content and has been 
tenned “lime-induced.” The typical 
chlorosis of pineapples in Hawaii oc- 
curs on soils high in manganese and 
has l^cen corrected by the use of sprays 
that contain iron. 

Magnesium deficiency causes a chlo- 
rosis that first affects the older leaves. 
Magnesium is a component of chloro- 
phyll. Sand drown, the distinctive chlo- 
rosis of tobacco, is caused by mag- 



iNVIRONMENTAL, NONRARASITIC INJURIES 


nesium deficiency. The lowermost 
leaves of the plant first lose their nor- 
mal green color at the tips and margins 
and between the veins. The primary 
and secondary veins and the nearby 
tissue tend to retain the normal green 
color long after the remaining leaf 
tissue has l:)ecome pale green or almost 
white. The deficiency rarely occurs 
until the plants have attained consid- 
erable size. It is called sand drown be- 
cause it is more prevalent in deep, 
sandy soils and during seasons of ex- 
cessive rainfall. Corn has streaks on 
the lower leaves when magnesium is 
deficient. The cotton plant shows 
chlorosis, and a purplish-red color de- 
velops in the yellow areas. Leaves of 
vegetables turn yellow and die. Citrus 
trees develop a chlorosis known as 
bronzing. Leaves of deficient apple 
trees turn yellow and unhealthy and 
drop if the shortage is acute. 

Chlorosis and necrosis of young 
leaves are early symptoms of manga- 
nese deficiency. Tomato plants grow- 
ing in calcareous soils in Florida showed 
retarded growth, failure to blossom, 
chlorosis, and a necrotic spotting of 
the younger leaves until manganese 
was supplied. The gray speck disease 
of oats is due to too little manganese. 
The first seedling leaves of the oat 
plant arc of a normal green; later 
leaves are faintly yellow and develop 
necrotic spots. “Pahala blight” of 
sugarcane arises from too little man- 
ganese. Snap beans show a chlorosis of 
the young leaves; each new leaf shows 
more chlorosis, and affected plants 
finally die. Young leaves of an ailing 
tobacco plant lose color in even the 
smallest veins; the contrast between 
the green and yellow places gives a 
checkered effect to the leaf. Chlorotic 
leaves develop small lifeless spots, 
which may enlarge and fall out. The 
spots arc distributed over the leaf — 
not only at the tip and margin, as 
with potassium deficiency. The acidity 
or alkalinity of the .soil on which the 
plant grows appears to dominate man- 
ganese absorption, as most instances 
of deficiency of manganese have been 


97 

reported on neutral or alkaline soils. 

The effect of molybdenum deficiency 
was first mentioned as a cause of the 
whiptail disease of cauliflower in New 
Zealand and Australia. The disease 
restricts the development of the leaf 
lamina, so that sometimes the midribs 
are left bare. In extreme cases the 
growing point dies. The effects of a 
shortage of molybdenum on tobacco 
and tomatoes has been reported for 
plants grown in nutrient solutions. 
The tomato plant shows a mottling of 
the lower leaves, followed by necrosis 
and crinkling. The fruit set is poor be- 
cause most of the blossoms shed. 
Tobacco show's much the same symp- 
toms when molybdenum is deficient; 
the shedding of flower buds leads to a 
reduction in amount of seed. Various 
crops, particularly legumes, have re- 
sponded favorably to the addition of 
molylxlenum on serpentine and iron- 
stone soils in some areas. 

Shortage of nitrogen, perhaps the 
most common of the deficiencies, 
show's up at any time from the seed- 
ling stage to maturity. First the plant 
loses its normal green color. The growth 
rate slows down. Then lemon, orange, 
red, or purple tints develop and the 
older leaves dry or drop. Leaves that 
develop later when nitrogen is trans- 
ferred to them from the older leaves 
are small; the production of fruit or 
seed is correspondingly reduced. The 
growth of nitrogen-deficient plants is 
sparse, spindly, and erect. The roots 
may be long and little branched; the 
twigs of trees are short and small. 
Small grains show a marked reduction 
in number of tillers and consequently 
yield poorly. Not all effects of nitrogen 
deficiency are bad, however. The 
growth of broad leaved plants, such as 
tobacco, can be regulated by with- 
holding nitrogen to produce leaf of a 
certain type, such as the bright lemon- 
colored leaf known as the flue-cured 
type. Fruit trees on nitrogen-deficient 
soil may produce highly colored fruits 
that store well. 

Shortages of phosphorus lower plant 
grpwth markedly. The symptoms arc 



YEARBOOK OF A« R I C U L T U I E I B S 3 


98 

not always clearly defined. Usually 
the leaves are small and erect, the lat- 
eral buds are few, and the roots may 
be sparsely branched. But most of the 
effects of phosphorus deficiency ap- 
parently are more general. The leaves 
usually are dark green, but in later 
stages or in extreme cases they may be 
dull green and may show purplish 
tints. Sometimes necrosis is evident. 
When the older leaves dry up or shed 
they are dark brown to almost black. 
The cereals often show purpling on 
older leaves. Tobacco leaves are a 
dark gray green and maturity is de- 
layed. Production of fruit and seed is 
reduced and slow. 

Chlorosis, commonly beginning on 
the older leaves at their tips and mar- 
gins, is typical of potassium deficiency. 
Necrosis follow’s, first as small areas 
that gradually enlarge and merge. The 
dead areas may fall out so the leaves 
get a ragged appearance. Grasses, 
when potassium is deficient, show a 
yellowish streaking which, on older 
leaves, may develop into .scorching. 
The stalks on such plants arc short, 
roots arc poor, and the ears arc poorly 
filled at the tip. The tobacco plant 
becomes bluish green, mottled, and 
chlorotic. Generally the lower leaves 
show the first symptoms, but if the 
shortage operates during later growth 
stages cf the plant, when growth is 
rapid, the upper leaves may show the 
first symptoms. Mottling is followed 
quickly by necrotic spotting at the 
leaf tips and margins between the 
veins. Tomato and the potato plants 
show much the same symptoms as 
tobacco. Tomato fruits fail to ripen 
evenly; often greenish-yellow patches 
are intermingled with the red of the 
red-fruited varieties. Colton and the 
sweetpotato develop chlorosis and 
necrosis of the older leaves and some 
leaf shedding. Cotton rust is a.ssociated 
with potassium deficiency. Foliage of 
deciduous fruit trees becomes bluish 
green; intervein chlorosis, necrosis, 
and marginal scorch occur on older 
leaves; extreme cases involve dieback 
of shoots and branches, and fruits of 


poor quality. Citrus fruits display small 
leaves, fluting or tucking along the 
midrib, small and poor fruit, and die- 
back in serious disorders. 

The effects of sulfur deficiency on 
plant growth generally resemble those 
caused by shortage of nitrogen. The 
younger leaves display a pronounced 
yellowing with little or no drying of 
older leaves. While nitrogen shortage 
4s accentuated and sometimes brought 
about by excessive rainfall, sulfur 
shortage is often more apparent during 
dry periods and in dry areas, since 
sulfur dioxide, a common air contami- 
nant, is brought down by rainfall. 
Grasses lose their normal green color 
when sulfur is withheld. Leaves of 
legumes become yellow and develop 
brown spots; the plants are less 
succulent and have thin stems. The 
tobacco plant first shows light-green 
leaves; veins and the tissue between the 
veins lose their green color w'hen sulfur 
is deficient. Much the same symptoms 
have been reported for the tomato. 
The tobacco plant recovers quickly 
from sulfur deficiency in times of ade- 
quate rainfall. Citrus trees show a 
marked yellowing of the younger 
leaves in the early stages of the 
deficiency. Some dieback of the twigs 
may occur later. A disease known as 
lea yellows is caused by .sulfur de- 
ficiency. The initial .stages of sulfur 
deficiency generally are marked by a 
yellowing of the younger leaves. When 
the condition becomes acute and last- 
ing, the older leaves may turn yellow. 
Leaves of citrus and the tea bush may 
die back. 

The initial effects of zinc deficiency, 
most evident on the older leaves of 
many plants, arc chlorosi.s, necrosis, 
shedding of leaves, and, in extreme 
cases, dieback of twigs in trees. Pecan 
ro.sette, citrus mottling, littlelcaf, and 
citrus rosette arc due to zinc defi- 
ciency. The corn plant shows yellow 
streaks on the older leaves when zinc 
is deficient. The streaks later become 
necrotic; the young leaves unfolding 
in the bud may be white or yellow and 
produce the white-bud disease. Sugar 



ENVIRONMiNTAL. NONPARASITIC INJURIES 


beet and potato develop leaf spots on 
the older leaves. The leaves of the 
potato are thickened and curled. To- 
bacco grown in purified, zinc-free sand 
or solutions in the greenhouse has 
shown deficiency symptoms — first a 
faint chlorosis on the older leaves at 
the leaf tip and margins between the 
veins. Necrosis soon develops as small 
areas that rapidly enlarge to involve 
the veins or the entire le^. The leaves 
are thick, internodes are short, and the 
corolla appears to be shortened. 

It is not possible to distinguish 
clearly between the disorders that are 
due to a deficiency as such and the 
disorders that are due to too much of 
another element. A deficient supply 
of one element implies an excess of 
other elements. A mass-action effect 
may arise as when too much of one ele- 
ment may interfere with the solubility, 
absorption, and utilization of another 
clement to the extent of developing 
acute deficiency effects. The effects of- 
ten may result from the acidity or alka- 
linity of the culture medium. 

Many of the nutrient elements in ex- 
cess may cause symptoms of toxicity. 
For example, boron in any consider- 
able amounts results in a marginal ne- 
crosis of the Older leaves followed often 
by stunting and death. Those symp- 
toms often have been seen following 
the use of irrigation water that car- 
ried toxic amounts of boron. The use of 
potash Salts in fertilizers containing ex- 
cessive amounts of boron has caused 
serious losses. 

Calcium, if present in amounts that 
cause alkalinity of the soils when the 
levels of iron, manganese, boron, or 
zinc arc low, often results in defi- 
ciency symptoms previously described 
as typical for the deficiency of each of 
those elements. 

An excess of copper causes necrosis, 
wilting, reduced growth, and death of 
plants. 

Too much iron may induce a defi- 
ciency of phosphorus or manganese. 

Magnesium present in large amounts 
may accentuate potassium deficiency 


99 

if the potassium supply is low. An ex- 
cess of magnesium may operate in 
much the same manner for calcium 
and show the calcium-deficiency symp- 
toms. 

Manganese when present in excess 
may bring about iron deficiency. Man- 
ganese is often present in acid soils in 
amounts sufficient to reduce plant 
growth. Low calcium is often associ- 
ated with this condition so that plant 
growth will be improved by liming to 
neutralize soil acidity. 

Excess amounts of nitrogen stimulate 
excessive growth and frequently may 
cause a deficiency of another element 
that is present in small amount. Often 
the other element is potassium; then 
the plant is commonly more susceptible 
to rusts (in the case of the small grains 
and cotton) and to leaf spots (tobacco). 
Sulfur often is added to acidify alkali 
soil to improve plant growth. Some- 
times the continued use of ammonium 
sulfate and other sulfates brings about 
a low pH, which often is unfavorable 
to plant growth. Excess sulfates bring 
into solution the extra manganese and 
aluminum that may be present in the 
soil and thus injure plant growth. 

Actually, most efiects associated with 
soil reaction are related to solubility 
of nutrient or toxic ions rather than 
injuries associated with the hydrogen 
ion. Since the acidity or alkalinity of 
soils in humid areas may range be- 
tween pH 4.0 and 8.0, most plants can 
grow successfully at that range if there 
arc no complications in regard to 
availability and toxicity of the ions 
present. The so-called alkali soils show 
a much higher pH and present a 
different problem. 

Salinity is a serious problem in 
many arid areas and in places where 
ocean spray or salt water floods agri- 
cultural soils. Excessive fertilization 
with soluble salts sometimes causes 
much the same kind of injury. The 
effect may vary according to the plants 
in question and the salts and concen- 
trations that are involved. Sometimes 
the effect is merely one of concentra- 



lOO 


YiAKBOOK OF AOIfCUirUlf 1959 


tion of soluble salts, but again, if alkali 
salts are present in excess, the soils are 
said to be alkali. The actual salts 
present may vary, but the three com- 
mon ones are sodium chloride (table 
salt), sodium sulfate (Glauber’s salt), 
and sodium carbonate (sal soda). Vari- 
ous other salts of sodium, calcium, 
magnesium, and potassium may also 
be present. 

Most of our common crop plants are 
sensitive to salts. Seed germination 
may be retarded or prevented. Young 
seedlings may die. When the plants do 
survive, the growth rate commonly is 
slow, the plant wilts, and the leaves 
bum at the tips and margins. Fruit or 
shade trees may survive for a time on 
saline soils and show chlorosis, pos- 
sibly caused by induced shortages of 
iron brought about by alkaline soil 
conditions. Their growth is reduced, 
leaves drop, and eventually the trec.s 
die. If conditions arc not too severe, 
the more resistant types of plants may 
have drought-resistant characteristics. 
The leaves arc small and have a 
thicker cuticle. Waxy coatings are 
more developed and the breathing 
pores are sunken below the outer 
surface, so that evaporation or tranS' 
piration arc reduced. 

Among the crop plants, sugar beets, 
Rhodes grass, and Bermuda-grass have 
the strongest tolerance to saline con- 
ditions. Those with medium-strong 
tolerance to saline conditions arc al- 
falfa, cotton, kale, barley as a hay crop, 
rape, and sorgo. The crops with me- 
dium tolerance include onions, .squash, 
flax, Ladino clover, sunflowers, rice, 
and rye as a grain crop. Those with 
the weakest tolerance include red clo- 
ver, snap beans, navy beans, vetch, 
and wheat as a grain crop. 

J. E. McMurtrey, Jr., ts principal 
physiologist and project leader of investiga- 
tions of production^ breeding, disease, and 
quality of tobacco in the Bureau of Plant 
Industry, Soils, and Agricultural Engineer- 
ing, He is stationed at Beltsville, Aid, Dr, 
McMurtrey has degrees from the University 
of Kentucky and the University of Maryland, 


The Effects 
of Soil 
Fertility 

George L, McNew 

Many farmers and gardeners have 
observed that plant diseases have be- 
come more prevalent and soils less 
fertile than they used to be. Some have 
argued that the two phenomena are 
related and that diseases are more de- 
structive because the plants have been 
weakened by poor mineral nutrition. 
A few have even maintained that there 
would be no serious disease problems 
if plants were grown in properly con- 
ditioned soU, but evidence obtained 
in hundreds of experiments throughout 
the world docs not uphold this extreme 
viewpoint. 

Soil fertility does affect the preva- 
lence and severity of some plant dis- 
eases, but it is only one of several 
factors that predispose plants to infec- 
tion by fungi, bacteria, vimses, and 
nematodes. We can make no sweeping 
generalizations about the effect of 
fertilizers on diseases because of the 
extreme differences in crop plants, 
their special nutritional requirements, 
the soil types upon which they are 
grown, and the diversity of the patho- 
gens that attack them. Some diseases 
are severe on weakened, undernour- 
ished plants. Many others are most 
destructive when plants arc growing 
vigorously. 

If wheat on a moderately fertile soil, 
for example, is given an extra supply 
of nitrogen it probably will escape 
seedling diseases more readily, be 
more subject to pythium root rot, 
suffer less from take-all disease, and 



THe tffSCTS OF SOIL FERTILITY 


lOI 


be more subject to infection by leaf 
rust and powdery mildew. Phosphorus 
and potassium fertilizers would have 
an entirely different effect on those 
same diseases. The addition of barn- 
yard manure to wilt-infested cotton- 
fields will reduce the amount of wilt 
in places in Arkansas where potassium 
and nitrogen are deficient but will 
increase the severity of the same disease 
on the delta of the Nile, where nitrogen 
is readily available. 

Such effects of soil fertility on plant 
diseases must be understood in this 
age when fertilizer practices are being 
changed so rapidly. More than i8 
million tons of commercial fertilizer 
and 2f) million tons of agricultural 
lime were used in 1 948. That is about 
twice as much as was required before 
the Second World War. Less barn- 
yard manure is being added to the 
soil each year. Not enough green 
manure from cover crops is being 
plowed under to maintain the organic 
matter content of soils on most of our 
farms. Under such conditions, both 
the balance of the various nutrient 
elements and the total supply of 
nutrients available to the plant can 
be expected to fluctuate appreciably 
from year to year and during the 
growing season. 

The primary consideration in 
fertilizing soil is to use such materials, 
in combination with suitable crop 
rotations and other soil management 
practices, as are necessary to promote 
the maximum productivity of the 
plant. Disease control is a secondary 
consideration beyond the fact that 
one must avoid certain conditions, 
such as an excess of nitrogen or other 
available nutrients, a deficiency of 
potassium, or changes in soil reaction 
that affect diseases of a crop. 

Fertilizer materials that leave acid 
residues (such as those from am- 
monium sulfate, potassium sulfate, 
sulfur, and calcium sulfate) should 
be employed where neutral or alka- 
line soils favor diseases like potato 
scab. 


Sodium nitrate, calcium phosphate, 
limestone, and similar materials that 
leave alkaline residues should be 
used where diseases such as club- 
root of cabbage and some wilt diseases 
are suppressed by alkaline soils. 

Residues from organic matter are 
valuable for stimulating the growth 
of beneficial micro-organismS in the 
soil. They may destroy or prevent the 
growth of some plant parasites that 
arc not well adjusted to living in the 
soil environment. 

When diseases become a serious 
problem on properly nourished, pro- 
ductive plants, other control measures 
such as spraying, crop rotation, or 
use of disease-escaping varieties will 
have to be employed. There is no 
sound reason for starving the plant 
into an unproductive state in order 
to escape disease. If the plant is 
properly nourished and capable of 
full development, the disease control 
measures, such as spraying or soil 
disinfestation, are fully justified. They 
are a form of crop insurance. They 
pay the largest dividends on produc- 
tive soils. Often they arc not worth 
applying to weak and undernourished 
plants. 

Several plant diseases arc influ- 
enced so seriously by soil deficiencies 
that much of the damage from them 
can be avoided by soil treatment. 
The outstanding examples, which 
are discussed here, are take-all disease 
of wheat, wheat root rot, Texas root 
rot of cotton, sugar beet seedling 
diseases, fusariam wilt of cotton, wild- 
fire di.scase of tobacco, clubroot of 
cabbage, common scab of potatoes, 
bacterial leaf spot of peaches, and 
powdery mildew and rusts of cereals. 

A deficiency of any plant nutrient 
may influence disease development. 
Nitrogen, phosphorus, and potassium, 
the primary elements, are mentioned 
most often. The secondary elements, 
calcium, sulfur, silicon, manganese, 
and boron also have been observed 
to exert appreciable influence on the 
prevalence of plant diseases. 

Nitrogen is .supplied in the form of 



102 


YEARBOOK OF AGRICULTURE 1953 


sodium nitrate (Chilean nitrate), am- 
monium sulfate, urea, organic nitix)- 
gen, or anhydrous ammonia. It pro- 
motes vigorous growth and is essen- 
tial for production of amino acids, 
growth regulants, and new proto- 
plasm. Us^ to excess, it encourages 
rank, vegetative growth, delays ma- 
turity, and tends to cause thin cell 
walls. Fungi may penetrate the thin 
walls more readily than normal ones. 
Infected plants collapse more easily. 
Cereal plants lodge. Lesions on leaves 
elongate rapidly. Because nitrogen 
in a proper combination with other 
nutrients often speeds up growth of 
seedlings and roots, the plants escape 
severe damage from pathogens that 
develop slowly. But because nitrogen 
may prolong vegetative growth, leaves 
are exposed to infection over a longer 
season. The roots, water-conducting 
tissues, leaves, and fruits of plants 
that are supplied with nitrogen are 
more nutritious to most pathogens, 
which grow better in them than in 
nitrogen-deficient plants. 

Phosphorus is applied as rock phos- 
phate, superphosphate, ammonium 
phosphate, basic slag, or bonemeal. 
Phosphorus is essential for utilization 
of carbohydrates and for cell division 
because it combines with carbohydrate 
materials to form nucleic acids. Ade- 
quate supplies of phosphonis promote 
root growth and seed development. 
Applications of it are most beneficial 
against seedling diseases and certain 
root rots where vigorous development 
of roots permits the plants to escape 
destruction. It is essential for multipli- 
cation of viruses in host cells and may 
increase susceptibility to viruses and 
other disease agents if too abundant. 
Because of its use in building new cells, 
any imbalance with nitrogen may 
cause disease losses to increase. 

Potassium (potash) is supplied to 
soils as potassium chloride (Kainit, 
muriate of potash), potassium sulfate, 
potassium nitrate, or wood ashes. Un- 
like other essential nutrients, it does 
not become a structural part of the 
plant cell. It is a mobile regulator of 


cell activity and promotes the reduc- 
tion of nitrates and the synthesis of 
amino acids from carbohydrate and 
inorganic nitrogen. Potassium pro- 
motes the development of thicker outer 
walls in the epidermal cells and firm 
tissues, which are less subject to 
collapse. 

A deficiency of potassium enforces 
the accumulation of carbohydrates 
and inorganic nitrogen in the plant. 
Eventually it retards photosynthesis 
and production of new tissues. More 
plant diseases have been retarded by 
use of potash fertilizers than any other 
substance, perhaps because potassium 
is so essential for catalyzing cell activi- 
ties. It is unavailable in many soils of 
light texture because its salts are so 
water-soluble they readily leach from* 
the soil. 

Calcium is added to the soil as 
ground limestone, hydrated lime, gyp- 
sum, or calcium phosphate. It is es- 
sential for normal growth since it 
regulates chromosome development 
in cell division and is assimilated into 
the middle lamellae of new cell walls. 
It is therefore important for cell divi- 
sion and cell development. It also may 
neutralize acid byproducts of cell 
metabolism that could become harm- 
ful if not precipitated in an insoluble 
condition. Calcium also influences 
plant diseases indirectly by its cfl'ect on 
soil acidity, by neutralizing toxins pro- 
duced by wilt-inducing fungi, and by 
affecting cell division in those diseases 
where abnormal growth of tissues is 
important. Its balance with pota.ssium 
becomes a primary consideration in 
gall development because both mate- 
rials contribute to the growth and divi- 
sion of cells. 

Silicon affects the availability of 
potassium. It also may be combined 
with other materials to give the cell 
walls greater structural strength. The 
primary effect on plant diseases ap- 
parently is in the prevention of infec- 
tion by powdery mildew, a disease in 
which the fungus develops externally 
and usually penetrates the host cell 
through the outer wall. Sulfur is oxi- 



THI EFFECTS OF SOU FERTILITY 


dized to sulfates and thereby promotes 
soil acidity, which discourages growth 
and survival of some bacteria and 
fungi. 

Soil reaction (the hydrogen-ion con- 
centration, expressed in terms of pH 
units) influences the growth and per- 
sistence of some fungi and bacteria. 
The parasitic organisms that depend 
on delicate bodies, such as thin-walled 
swarmspores, may have difficulty in 
multiplying and infecting roots in soils 
at unfavorable hydrogen-ion concen- 
trations. Hydrogen-ion concentration 
may change the ability of pine tree 
roots to resist invasion, but that, if 
true, is an exceptional case. Soil reac- 
tion also may affect the availability 
of essential nutrients in the soil and 
the biological balance between plant 
parasites and saprophytic soil-inhabit- 
ing fungi and bacteria. 

Although organic matter contributes 
essential nutrients to the crop, it 
probably exerts more influence through 
the physical and biological changes 
diat it brings about in the soil. The 
carbohydrates and proteins in animal 
and plant byproducts provide nourish- 
ment to soil organisms that fix nitro- 
gen from the air, tie up available 
nitrates in the soil, and frequently 
suppress plant parasites by antibiotic 
activity. 

The humus from lignified plant tis- 
sues and other residual products pro- 
motes the massing together of soil 
particles and thereby improves aera- 
tion and water-holding capacity of 
soils. Organic matter may promote 
the growth of plant parasites or even 
facilitate their dispersal. Some of the 
decomposition products may increase 
the susceptibility of roots to invasion. 

Young seedlings of most crops are 
attacked by fungi that live in the soil or 
arc carried on the seed. 

Fungi such as Pythium deharyanumj 
P, ultimum, and Rhizoctonia solani live 
in most agricultural soils since they 
compete successfully for space with 
other soil saprophytes. They are stimu- 
lated to growth by organic matter 


103 

around the germinating seed. If seed 
germination or seedling development 
is delayed by excessive moisture or 
low temperature, the fungi may invade 
the seed or girdle the young shoot 
before the plant can establish itself. 
These fungi become progressively less 
aggressive as the plant begins to syn- 
thesize its own food and its under- 
ground tissues become lignified. 

Sugar beet seedlings arc particularly 
susceptible to these damping-off fungi 
and Aphanomyces cochlioides. Failure to 
secure a suitable stand occurs in 
Montana, Colorado, Iowa, Ohio, and 
elsewhere whenever heavy rains fall 
after seeding. The usual preventive 
mea.sure employed is to coat the seed 
with a fungicidal chemical, which 
prevents seed decay and protects the 
seedling near the seed ball. Seed dress- 
ings do not always prevent the post- 
emergence damping-off if the weather 
conditions arc favorable for the soil- 
inhabiting fungi. 

Much of the damage can be avoided 
by fertilizing the soil so as to promote 
vigorous growth of the seedlings. 
Phosphates alone at the rate of 400 
to 800 pounds an acre in moderately 
fertile soils are decidedly beneficial in 
preventing severe losses from Aphano- 
myces cochlioides at low soil temperatures. 
In Montana, where seedling losses of 
75 percent were common on a soil 
of low fertility, the application of 
manure, superphosphate, and sodium 
nitrate reduced losses to 21 percent 
and increased yields of roots from 7.47 
tons to 18.23 tons. 

The application of potash or nitro- 
gen alone is not effective. Moderate 
improvements have been obtained 
from manure alone. Manure and phos- 
phate or a combination of manure, 
phosphate, and nitratt: are definitely 
superior. Such treatments combined 
with a desirable rotation such as beets- 
potato-oats or a 3-year cycle of beets- 
alfalfa-oats-potatoes eliminated most 
of the seedling diseases in the experi- 
ments in Montana. 

Attempts to apply fertilizers as a 
seed dressing have not been uniformly 



YKAIBOOK OP AOIICUITUR6 1P53 


104 

successful. Highly soluble materials in- 
jure the young plants, and phosphate 
on the se^ is not so effective as it is in 
soil treatments. 

Although fertilizer treatments have 
not proved so successful on other 
crops, there are reports of benefits 
from fall application of balanced 
fertilizers to clover seedlings in Russia 
and from calcium on soybeans in the 
United States. Nitrogen applied to 
cucumber seedlings reduced the se- 
verity of damping-off, presumably by 
accelerating the lignification of under- 
ground portions of the stems. Most 
observers have reported, however, 
that application of nitrogen alone 
or in excessive proportions makes 
seedlings more susceptible to attack. 
The damping-off of pines, other coni- 
fers, and cotton is increased by 
nitrogen fertilizers. Sodium nitrite, 
however, applied at the rate of 4 to 8 
ounces to the square yard, several 
weeks before planting, destroys nema- 
todes as well as damping-ohf fungi and 
then oxidizes to harmless nitrates, 
which may be used as a source of nitro- 
gen by the young plants. 

Australian w'orkers reported that 
seed decay of peas was more severe 
in poor soils than in fertile ones, but 
emergence of peas was not improved in 
Colorado by application of different 
fertilizers to poor gravelly soil. Seed 
decay of peas may be increased by 
applying fertilizers in direct contact 
with seed. The injury can be attributed 
largely to the water-soluble nitrogen 
in balanced fertilizers, because phos- 
phorus alone causes little damage and 
potassium salts are only slightly in- 
jurious. Decay increases in proportion 
to the percentage of nitrogen in the 
formula and the total amount applied. 

Tomato seedlings arc also more 
subject to damping-off when soluble 
salts accumulate around the tender 
stem and roots. The concentration of 
salt seems to be more important than 
the kind of compound, except that 
potassium salts are tolerated in fairly 
heavy concentrations. 

. A severe ^oot rot of wheat in Sas- 


katchewan and other areas on the 
Great Plains, known as the browning 
disease, is caused by Pythium arrkewh 
manes. The same fungus attacks sugar- 
cane in Louisiana and Hawaii. Studies 
made on the two crops have tended to 
confirm each other on the effects of 
soil fertility. 

The disease is most severe on sugar- 
cane and wheat in poorly drained, 
wet soils. Such soils have many 
products of anaerobic respiration, 
such as salicylic aldehyde, present in 
concentrations of 50 parts per million 
or more. Salicylic aldehyde is ordi- 
narily oxidized by species of Penicillium 
and Actinomyces in wcll-acraled soil, 
but the grow'^th of these fungi is 
suppressed by the anaerobic conditions 
in wet fields. The salicylic aldehyde 
does not affect the growth of either 
the roots or the root-rotting fungus, 
but it does predispose plants to attack. 
Injurious concentrations of salicylic 
aldehyde .may be avoided by draining 
the soil or using fertilizers that pro- 
mote oxidation. 

Wheat plants that have severe root 
rot usually contain nitrogen in ade- 
quate to excess amounts but are 
deficient in phosphorus. The disease 
is particularly severe on fields that 
were held under fallow cultivation 
the preceding years. The nitrogen- 
fixing bacteria increase the supply of 
nitrates during the fallow period and 
these are readily available to the 
next crop. Severe damage from root 
rot can usually be avoided by adding 
phosphates to the soil or by reducing 
the supply of nitrates available so as 
to restore a normal balance between 
the two nutrients. 

Wheat straw, plowed under, stimu- 
lates the growth of soil micro-organ- 
isms, which assimilate the available 
nitrogen and thereby reduce the 
amount immediately available to the 
new wheat crop. So the balance with 
phosphorus is temporarily restored dur- 
ing the period when the young plant is 
most susceptible. The same result is 
achieved in Hawaii by applying to the 
soil press cake from the cane extracting 



THE EFFECTS OF SOIL FERTILITY 


plant or crude cane sirup wastes. Be- 
sides promoting growth of soil bacteria, 
the materials provide extra potassium, 
which also helps reduce loss from root 
rot. Barnyard manure, swcetclovcr, or 
weed hay may also be used to advan- 
tage. 

Nitrogen added alone to infertile soil 
increases the susceptibility of roots to 
invasion by lithium. The plants are not 
capable of developing new roots and 
consequently are severely injured. 
When phosphorus and nitrogen are 
added simultaneously the roots are still 
susceptible to infection, but new growth 
develops so rapidly that the plant es- 
capes severe damage. The ability of 
plants to produce new roots explains 
the beneficial effect from balanced 
fertilizers. 

Several other root rot and foot rot 
diseases of cereals are caused l )y species 
of Fusariwn, Cercosporella, and Helmin- 
thosporium. The diseases usually are 
found to be most severe on dwarfed, 
undernourished plants or on plants re- 
ceiving an excess of nitrogen. Field 
tests with phosphates and balanced 
fertilizers, however, have failed to pre- 
vent losses in many localities. In at 
least one locality in Canada excess sol- 
uble salts in the soil increased the 
severity of iiclrainthosporium and fu- 
sarium root rots. 

The take-all disease of wheat 
causes severe losses in the United 
Slates, Canada, and Australia. Much 
of the damage can be avoided by main- 
taining a proper balance of soil nutri- 
ents. The soil requirements seem to 
differ in different areas. Decided bene- 
fits from nitrogen fertilizers have been 
obtained in England and Canada. 
Phosphates have proved beneficial in 
Kansas and Australia. 

Some of the benefits from .soil fertili- 
zation have been phenomenal. In an 
Arkansas field where 8o percent of the 
plants were infected, on an unfertilized 
plot, the application of lo tons of barn- 
yard manure or 400 pounds of 4-8-3 
fertilizer to the acre reduced infection 
to 45 and 7 percent, respectively. 


105 

Experiments in sand cultures have 
shown that a suitable balance among 
nifrogen, phosphorus, and potassium 
must be maintained if losses from take- 
all are to be avoided. Severe infection 
occurs when all three nutrients are in- 
adequate, or phosphorus and potassi- 
um are in short supply. Nitrogen may 
increase the incidence of root infection. 
As in pythium root rot, however, if 
phosphorus is also present the plant de- 
velops new roots to replace the infected 
ones and produces a good crop. 

Readily available nitrogen has three 
effects on the development of take-all. 
An adequate supply during the fall and 
winter permits the causal fungus, Ophi- 
obolus graminis^ to prolong its existence 
on infested stubble. Therefore the fun- 
gus often lives longer in soil under fal- 
low culture than in soils seeded to cover 
crops or oats, which utilize all available 
nitrogen. If nitrogen is made available 
to young plants, the roots arc more 
readily invaded. Adequate nitrogen, 
however, will also permit the rapid re- 
placement of diseased roots with new 
ones if phosphorus is present. 

Because of these various effects, S. 
D. Garrett suggested in the Annals of 
Applied Biology for 1948 that trefoil 
was the ideal cover crop to precede 
wheat. Plowed under in the fall, it 
stimulated growth of micro-organisms 
so all available nitrogen in the soil 
would be incorporated into their bod- 
ies and the survival of the take-all 
fungus would be reduced. The plant 
residues and micro-organisms would 
then decompose the following spring 
and summer to provide a continuous, 
uniform supply of nitrogen to promote 
disease-escaping ability in the wheat 
crop. 

For .some time it waf believed that 
organic matter reduced the severity of 
take-all by encouraging the multiplica- 
tion of soil micro-organisms that were 
antagonistic to Ophiobolus graminis. 
Many carbohydrate-rich materials do 
increase the soil flora, but most of their 
effect is now attributed to uniform 
supply of nitrogen and phosphorus. 
There is very little change in the popu- 



YEARBOOK OF AGRICULTURE 1953 


io6 

lation of bacteria and fungi in the 
immediate vicinity of the roots where 
the take-all fungus attacks and all ef- 
fective materials, such as chicken ma- 
nure, chopped ^falfa, horse manure, 
and barley or oat grain, increase the 
amount of available nitrate and phos- 
phate in the soil. 

Texas root rot has been one of the 
more' difficult diseases to control. 
There is little chance of obtaining 
disease-resistant varieties because the 
fungus attacks more than a thousand 
species of plants. As the fungus spreads 
slowly in the soil by mycelial growth, 
infested areas gradually enlarge each 
year. But it is a poor soil invader when 
divorced from plant roots and does not 
compete well with other soil-inhabiting 
organisms. Consequently it often di- 
minishes after 2 or 3 years. 

The disease is most prevalent in 
alkaline soils. Its radial spread may be 
controlled by increasing the soil acidity 
through use of sulfur. The fungus 
grows better in soils made alkaline by 
calcium carbonate than in soils re- 
ceiving calcium nitrate, calcium sul- 
fate, or calcium phosphate. The bark 
of infected roots usually is rich in 
phosphorus but deficient in nitrogen. 

The disease can be controlled by 
adding nitrogenous fertilizers to the 
soil. The use of 15 to 20 tons of barn- 
yard manure or green manure an acre 
will permit production of a good crop 
on infested soil. Ammonium sulfate 
and balanced fertilizers such as 9 -3-3 
at 600 to 900 pounds to the acre have 
been recommended for silt loam soils 
of Texas. Phosphate fertilizers increase 
the severity of root rot. A potassium 
deficiency reduces slightly the severity 
of the disease. 

Many of the effects of fertilizers can 
be attributed to changes in the soil 
microflora that destroy the sclerotia — 
the resting bodies — of the pathogen 
and otherwise reduce its prevalence, 
The reduction of sclerotia is in pro- 
portion to the amount of organic sup- 
plements used over a range of 0.5 to 
5.0 percent in clay, sandy loam, or 


sandy soils. Crop refuse, green manure, 
and such materials as starch, cellulose, 
and peptone promote the destruction 
of the root rot fungus. If soil micro- 
organisms are not present, the organic 
matter alone will not destroy the 
sclerotia. 

The amount of carbohydrate in the 
root bark affects the severity of dis- 
ease. Because a moderate amount is 
necessary for invasion, young seedlings 
escape infection until they begin to 
synthesize their own foods. Later in 
the season carbohydrates begin to ac- 
cumulate in the bark and escape into 
the surrounding soil. Thus the growth 
of soil organisms, which apparently 
can inhibit the root rot fungus, is pro- 
moted. High carbohydrate levels in 
the roots induced by extra light, less 
growth of twigs, reduced set of bolls, 
or temporary deficiency of nitrogen 
lower the amount of infection. Further 
support for this observation has been 
obtained by studying the relationship 
of the soil organisms to the resistance 
of corn roots. Corn secretes carbohy- 
drates from the roots and is resistant 
under normal circumstances but be- 
comes susceptible when grown in the 
absence of soil saprophytes that can 
inhibit the fungus. 

The cotton wilt caused by Fusa^ 
rium vasinfectum can be controlled by 
the use of resistant varieties if nema- 
todes are not present in soil. The 
disease is most severe in the potash- 
deficient soils of the United States. 
It is very destructive on light, sandy 
soils such as the alluvial deposits along 
streams, where soluble nutrients have 
been lost by leaching. It is severe also 
on the fertile, heavy soils of the Nile 
Delta in Egypt, presumably because 
of the availability of nitrogen. Barn- 
yard manure has been known to re- 
duce the severity of cotton wilt in the 
United States since 1907, presumably 
because it corrects potash deficiency. 
Its use in E^t, however, has been 
reported to increase the severity of 
wilt. The disease may occur in soils 
ranging in reaction from pH 4.6 to 



THE EFFECTS OF SOIL FEETILITY 


8.4 but is most common on the more 
acid soils of Arkansas because most of 
them are deficient in potash. 

Wilt can be reduced by application 
of 20 to 100 pounds of potash to the 
acre^ but best results are had on most 
light soils from use of a balanced fer- 
tilizer, such as 6-8-8 or 4-10-7, at 
the rate of 300 to 400 pounds to the 
acre. On moderately fertile soils nitro- 
gen should be omitted or used spar- 
ingly because an excess of nitrogen 
will increase the severity of wilt. A 
surplus of phosphate may be harmful 
but usually is not so influential as the 
balance between nitrogen and potash. 
Ammonium nitrogen is more con- 
ducive to wilt than nitrate salts. 

Much of the wilting promoted by 
nitrogen fertilization can be avoided 
by using calcium nitrate in alkaline 
nutrient solutions or by plowing 
under green manure crops. 

The wilt of cotton caused by 
cillium alho-atrum does not respond to 
soil treatments in the same way as the 
fusariam wilt. Application of potassium 
does not reduce infection and may 
even increase the severity of wilting. 
Nitrogen treatments increase the se- 
verity of the disease. Tomatoes infected 
by the fungus respond to nitrogen and 
potassium fertilizers much as cotton 
does. On both crops, plant residues 
arc beneficial because they lower the 
availability ol soluble nitrogen com- 
pounds. 

Clubroot of cabbage and other 
cruciferous crops is caused by a fun- 
gus {Plamodiophora brassiccu!) that sur- 
vives for several years in infested soil. 
The fungus produces free-swimming 
swarmspores that infect roots and root 
hairs. The swarmspores are released 
most readily in neutral or acid soils 
and may be inhibited in alkaline 
soils. Therefore, and because cabbage 
can grow well in calcareous soils, 
many investigators have recommended 
that soils be limed to pH 7.0 to 7.2 in 
order to escape the disease. 

Turnip growers 200 years ago 
learned to use marl or alkaline clay 


107 

on their fields. Ground limestone or 
hydrated lime is commonly recom- 
mended today. Comparable amounts 
of gypsum and calcium chloride are 
relatively ineffective. Although the 
hydrogen-ion concentration of soils 
does affect clubroot, the disease has 
been observed in alkaline soils of pH 
7.8 to 8.1. Other contributory factors 
consequently must also be involved. 

The disease is most severe on cab- 
bage that gets abundant supplies of 
balanced nutrients. A deficiency of 
potassium suppresses infection on cab- 
bage, mustard, and turnip. The 
disease is most severe when there is a 
deficiency or excess of nitrogen or an 
excess of potassium. Infection occurs 
most readily on large, actively grow- 
ing roots, but mineral nutrients ob- 
viously influence the disease by chang- 
ing the physiological balance within 
the host tissue because nitrogen defi- 
ciency promotes infection on poorly 
nourished roots. 

Apparently the ratio of calcium 
to potassium may be more important 
than soil reaction in determining the 
severity of clubroot. Clubroot infec- 
tion diminishes on moderately fertile 
silt loam soil as the ratio of calcium 
to potassium is decreased either by 
adding calcium or using less potas- 
sium while the other constituent is 
held constant at nominal concentra- 
tions. The effects of calcium and 
potassium have been observed in 
acid, neutral, and alkaline soils, but 
the general severity of the disease 
increases as soils become more acid. 

The common scab of potato {Strep- 
tomyces scabies) is usually avoided by 
growing potatoes in acid soils. Scab 
rarely occurs in soils at pH 4.8 or 
below and is not serious at pH 4.8 to 
5.1. It increases in prevalence and 
destructiveness at pH 5.4 to 7.0. 
Potatoes grow best at pH 5.0 to 5.5, 
but tolerate a range of pH 4.6 to 6.i 
without loss of yield. The usual 
recommendation for potatoes on scab- 
infested soil is to adjust the reaction 
to pH 5.0 to 5.2. That may be done by 



I08 YEAIBOOK OF AGSICULTUIE 1953 


applying sulfur or using acidic fer- 
tilizers such as ammonium sulfate and 
potassium sulfate in preference to 
sodium nitrate and other materials 
that leave alkaline residues. 

The sulfur treatment, which is also 
used for the control of soil pox of 
sweetpotatoes caused by Streptomyces 
Apomoea^ consists of applying 500 to 
3,000 pounds an acre well in advance 
of planting so it will be converted to 
acid by the soil organisms. It may 
produce microbicidal concentrations 
of hydrogen sulfide. J. H. Muncie and 
his co-workers in Michigan reported 
in 1944 that the scab organism may 
become adjusted to the sulfur treat- 
ment and not be controlled by a 
second application even when soil 
is made acid. Heavy applications of 
sulfur may .suppress growth of pota- 
toes; so sulhir-tolerant crops such as 
cotton have been recommended for 
use the first year after treating the 
soil. In the Florida potato-growing 
areas, sulfur-treated soils receive an 
application of lime before planting 
to potatoes and related crops. 

Barnyard manure often increa^^es 
scab, but we do not fully understand 
the exact role of various nutrient 
elements. The ratio of calcium to 
potassium is important. One can 
assume that one of the major effects 
of hydrogen-ion concentration is on 
this rrtio because it affects the mo- 
bilization of cationic plant nutrients. 
In tests by R. A. Schroeder and W. A. 
Albrecht, of Missouri, where ex- 
changeable ions w'erc controlled by 
the colloidal clay technique, infection 
increased as supplies of either cal- 
cium or potassium were increased 
in the presence of an adequate supply 
of the other. Infection was about 
equally prevalent at pH 5.2 and 6.8 
when the calcium -potassium balance 
remained constant. Scab was least 
severe at either reaction when the 
two elements were balanced. Those 
observations were repeated and ex- 
tended by G. A. Cries, J. G. Horsfall, 
and H. G. M. Jacobson, of Connect- 
icut. 


The angular leaf spot and wildfire 
disease of tobacco is caused by a bae* 
terium {Pseudomonas tabaci) that in- 
vades the leaf through the stomata. 
Infection often is restricted to a small 
spot around the stomata unlc.ss the leaf 
tissues have become water-soaked 
either by driving rainstorms or root 
pressure during rainy periods. The 
disease is most severe in the South- 
eastern States in soils that are defi- 
cient in potassium. 

Potassium fertilizers do encourage 
thicker cell wails, heavier cuticle, and 
stronger mechanical tissues in tobacco. 
The leaves of such plants are less easily 
water-soaked. Consequently they are 
less likely to be seriously injured by 
the wildfire bacterium. Plants receiv- 
ing an excess of nitrogen have thinner 
cell walls and more succulent tissue 
and are more subject to water soaking 
and invasion. 

Balanced supplies of potassium and 
nitrogen arc essential if wildfire is to l)e 
avoided. As barnyard manure contains 
both elements, the application of 6 to 
8 tons an acre has been effective in 
Kentucky and other areas where IxJth 
elements are deficient. Balanced ferti- 
lizers like 4-10-6 or 6-2-3 are more 
effective than nitrogen and potassium 
alone in preventing wildfire. Most 
soils in the tobacco-growing areas re- 
quire alK)ut 40 to 60 pounds of potash 
an acre. The usual recommendation 
is for an application of 600 to 1,500 
pounds of mixed fertilizer. 

Peaches and plums frequently are 
defoliated in late summer by Xantho^ 
monas prunu The bacteria from over- 
wintering cankers on the twigs invade 
the leaf through the stomata and cause 
irregular circular lesions about one- 
quarter inch or more in diameter. The 
infected area dies and falls out, thereby 
producing a shot-hole appearance. 
Sometimes the infected leaves become 
yellow and fall from the tree; on other 
trees they continue to function. Usually 
the nitrogen-deficient leaves in the 
center of the crown and on the lower 
branches arc the first to fall. The most 



INi Iff ECTS OP 

severe damage occurs to trees on in- 
fertile, light, sandy soils. 

Vigorous, healthy leaves in an or- 
chard in Pennsylvania had more po- 
tassium than infected leaves. Applica- 
tions of potassium and magnesium, 
however, failed to reduce the severity 
of disease. Excellent results have been 
obtained from six applications of sodi- 
um nitrate each at the rate of i pound 
to a tree. The added nitrate does not 
prevent infection, but it does reduce 
defoliation so that trees retain their 
leaves throughout the summer and 
fall. 

W. D. Valleau concluded from his 
observations in Kentucky that trees 
supplied with adequate nitrogen have 
ability to excise infected spots and 
retain their leaves longer than under- 
nourished trees. 

Moderate applications of ammo- 
nium sulfate will reduce the bacterial 
canker of prunes, according to E. E. 
Wilson. The treatment did not prevent 
infection through the Icnticels but it 
did encourage the development of 
periderm and the cankers healed more 
promptly. The bacteria] canker of 
plums in England caused by Pseudo- 
monas moTSprunorum differs from the 
disease caused by Xanthomonas pruni in 
that nitrogen has no beneficial effects, 
the largest cankers occurring on plants 
receiving generous supplies of balanced 
fertilisers. The smallest lesions are oxi 
plants deprived of phosphorus. 

Powdery mildew of cereals is 
caused by highly specialized races of 
Erysiphe grafninis, an obligate parasite 
that lives predominantly on the sur- 
face of leaves and sends haustoria 
(rootlike appendages) into the epi- 
dermal cells, where they procure foods. 
Either mechanical resistance of the 
cell wall to penetration by the haus- 
toria or an undesirable nutritional 
balance inside the cell may restrict 
mildew development. 

Heavy application of nitrogen to 
soil promotes rapid growth of cereals 
and increases the severity of mildew. 
Because the outer walls of the epidcr- 


SOIl PEETIlltY 109 

mal cells in such plants are thinner, it 
has been assumed that that change 
facilitates infection. Thicker walls may 
be developed by balancing the nitro- 
gen with adequate supplies of potas- 
rium and phosphorus. The potassium 
increases resistance, but phosphorus- 
fed plants are more susceptible despite 
the thicker cell walls. Although the 
Sickness of cell wall undoubtedly 
influences the susceptibility of wheat 
and barley, other factors (such as the 
chemical composition of the cell con- 
tents) also regulate the development 
of mildew. 

Potassium has beneficial effects on 
cereals. Potassium silicate is particu- 
larly effective in improving resistance 
of wheat and barley but not oats. 
The outer cell membranes of the epi- 
dermis arc perceptibly thickened, 
presumably by depositing silicates and 
changing the functions of the cell 
protoplasm. Similar effects have been 
reported from use of silica gel on rye, 
barley, and wheat. Low concentrations 
of nitrogen and adequate supplies of 
potassium promote silicification of cell 
walls. Silicates, however, do not im- 
prove the resistance of rye to rust, 
oats to hclminthosporium blight, or 
corn to smut and anthracnosc. 

Secondary nutrients also influence 
resistance of cereals to mildew. A 
deficiency of boron in California and 
deficiency of manganese in Australia 
were found to increase disease. 

The cereal rusts consist of many 
physiologic races that arc intensely 
specialized in their parasitic behavior. 
They multiply when in contact with 
living cells and become dormant or 
die when the host cells die because 
they cannot utilize decaying organic 
matter readily. 

As early as 1903 J. C. Arthur com- 
mented: “So intimate is the associa- 
tion of parasite and host as a rule the 
vigor of the parasite is directly pro- 
portional to the vigor of the host.” 

If growth of the plants is retarded 
by too much nutrients, the fungus may 
be less fruitful. 



I lO 


YEARBOOK OF AORICULTURB 19S3 


The leaf rusts of wheat, oats, and 
corn arc influenced by mineral nu^- 
tion. Heavy supplies nitrogen in- 
crease the susceptibility of foliage when 
supplied through the roots or by leaf 
immersion. The nitrogenous materials, 
in descending order of effectiveness 
for accentuating rust, are ammonium 
nitrate, ammonium sulfate, ammoni- 
um chloride, urea, glycol, ammonium 
phosphate, magnesium nitrate, aspara- 
gin, calcium nitrate, potassium ni- 
trite, and sodium nitrate. Resistance 
is impaired the least by these materials 
when supplied with adequate carbo- 
hydrates. Excessively heavy applica- 
tions of sodium i^trate or potassium 
nitrate reduce infection of wheat. 

Potassium salts in moderate amounts 
increase the resistance of wheat, rye, 
and oats to leaf rusts. Moderately re- 
sistant varieties may be made suscep- 
tible by depriving them of potassium. 
The reaction of very resistant varieties 
is not affected materially and suscep- 
tible varieties do not become immune 
when supplied with potassium salts. 
Although phosphorus has little effect 
on the severity of leaf rusts, it may 
increase resistance slightly when other 
nutrients are available in adequate 
amounts. Heavier infection usually is 
promoted by increasing the amount of 
balanced nutrients available to the 
plant. 

The reason has not been fully ex- 
plained. Any deficiency that retards 
growth of the host may reduce the 
severity of rust infection, as was ob- 
served by E. B. Mains for calcium, 
iron, magnesium, potassium, phos- 
phorus, nitrogen, and sulfur defi- 
ciencies in the greenhouse. The albu- 
men content of leaves is increased by 
heavy nitrogen fertilization and low- 
ered by potassium; consequently it 
might regulate susceptibility. The pro- 
duction of a rust-inhibiting toxin by 
the plant may be affected. A. F. 
Parker-Rhodes found that extra nitro- 
gen or a deficiency of minor elements 
enhanced the ability of wheat to pro- 
duce a toxin for Puccinia glumarum. 
The toxins possibly are produced by 


the autolysis of proteins in the infected 
leaf. 

Black stem rust may be influenced 
by mineral nutrition of wheat, accord- 
ing to G. R. Hurdi. A moderate 
supply of nitrogen increases the in- 
cidence of stem rust. Heavy applica- 
tions weaken the straw and predispose 
the plants to severe damage from en- 
larged pustules and lodging. Potassium 
and, to a less extent, phosphorus in- 
crease resistance when a moderate 
supply of nitrogen is available. Cal- 
cium phosphate in the absence of 
nitrogen rendered plants very resistant. 
He believes the mineral nutrients af- 
fect resistance by changing the thick- 
ness of epidermal walls, the number of 
stomata, or the amount of reenforcing 
tissue (sclerenchyma), which limits 
the size of pustules. 

No change in the proportion of 
sclerenchyma to collenchyma that 
could be correlated with resistance was 
found by Helen Hart, of the University 
of Minnesota, except in the variety 
Kota during one season. Plants re- 
ceiving treble superphosphate were 
less severely infected and had a larger 
percentage of collenchymatous tissue. 
The pustules in the plants were small 
and well separated by sclerenchyma. 
Plants receiving ammonium phosphate 
or balanced nitrogen, phosphorus, and 
potassium had more collenchyma 
tissue. She concluded, however, that 
nitrogen probably did not increase the 
amount of susceptible tissue in most 
varieties of wheat. 

The type of reaction in very resist- 
ant or very susceptible varieties of 
wheat cannot be changed materially, 
according to J. M. Daly. The variety 
Thatcher, however, grown at 65® to 
75® F. in the greenhouse was com- 
pletely resistant to race 56 when sup- 
plied with ammonium nitrogen and 
partly susceptible when supplied ni- 
trate nitrogen. Mindum wheat was 
resistant regardless of nitrogen source. 
The reaction of Marquis, Mindum, or 
Thatcher under field conditions was 
not altered by different amounts of 
nitrogen. 



THE EFFECTS OF SOIL FEETILITY 


III 


No fundamental change in resist- 
ance of Wheat to Puccinia graminis was 
observed from use of fertilizers by 
£. C. Stakman and O. S. Aamodt in 
field trials that continued 8 years. 
Plants receiving nitrogen often had 
more infection, but that could be at- 
tributed to heavier growth of plants 
and delayed maturity. Their general 
conclusion was that the wheat farmer 
should give the soil the fertilizers that 
the wheat needs but avoid too much 
nitrogen; add potash and phosphates 
judiciously — ^and the best results arc 
likely to 1^ obtained. 

Plant viruses arc most evident in 
young, rapidly growing tissues and 
may be masked in older tissues, par- 
ticularly if they are exposed to in- 
tense sunlight. Because the virus 
increases only in the living cells of the 
plant, any change in host physiology 
might be expected to affect the mul- 
tiplication of viruses. Most, of the 
information available on effect of 
mineral nutrition has been obtained 
from studying various strains of to- 
bacco mosaic in hosts that develop 
local lesions or systemic mottling of 
the leaves. 

The yellow strain of tobacco mosaic 
has been studied extensively by E. L. 
Spencer on plants in sand culture and 
nutrient-deficient soil. The virus cre- 
ates local lesions most readily in 
Turkish tobacco supplied with abun- 
dant nitrogen, phosphorus optimum 
for vegetative development, and po- 
tassium at minimum concentration 
for full growth of the plant. 

Increasing the supply of nitrogen 
beyond the optimum for growth in- 
creases susceptibility if all growth of 
the plant is not inhibited by nitrogen 
toxicity. Phosphorus increases sus- 
ceptibility directly in proportion to 
plant growth. A moderate surplus of 
potassium suppresses susceptibility to 
infection. The effeex of the three 
nutrients on the development of sys- 
temic symptoms is not closely cor- 
related with susceptibility to initial 
infection. Moderate or heavy appli- 


cation of potassium or phosphorus 
delays incubation by 7 or lo days. 
Either a deficiency or excess of nitro- 
gen accelerates the appearance of 
systemic symptoms. 

Neither infection nor systemic spread 
of the virus was completely controlled 
by the growth rate of the host induced 
by supply of nutrient. The local lesions 
formed on Nicotiana giutinosa, however, 
depend on vigorous growth of the 
host. A sevenfold increase in lesions 
occurred when ammonium phosphate 
was added to depleted soil. 

F. C. Bawden and B. Kassanis 
confirm that observation. They report 
greatest susceptibility in N. tabacum 
and JV*. glutinosa to tobacco mosaic and 
tomato aucuba mosaic when plants 
were supplied nitrogen and phos- 
phorus in concentrations optunum 
for plant growth. They feel phas- 
phorus is more important than 
nitrogen or potassium in regulating 
host susceptibility. 

As the virus protein contains both 
nitrogen and phosphorus, the two 
elements might be expected to control 
its multiplication. E. L. Spencer found 
that Turkish tobacco that had ample 
supplies of nitrogen had about 80 
times as much virus as nitrogen- 
deficient plants. Furthermore, the 
total virus content of deficient plants 
apparently declined after initial sys- 
temic development. Multiplication of 
virus did not depend on growth of the 
host since the virus multiplied rapidly 
in plants retarded by excessive nitro- 
gen supply. The vims concentration 
remained almost constant in infected 
plants that were deprived of nitrogen 
after inoculation. Apparently, there- 
fore, the host did not assimilate virus 
protein and the virus cannot multiply 
at the expense of protein in nitrogen- 
starved host cells. 

These studies were based on the 
infcctivity of extracted juice and may 
have to be modified when virus pro- 
tein is precipitated and measured 
directly. Spencer reported that virus 
preparations from young lesions puri- 
fied by ultracentrifugation had less 



112 


YEAtBOOK OE AOBICULTURE 1953 


intrinsic infectivity than virus frdtn 
older infections. This increase in 
infectivity did not occur when plants 
were deprived of nitrogen shortly 
after inoculation. These observations 
were not confirmed by F. C. Bawdi^n 
and B. Kassanis, who suggested that 
Spencer may have recovered some- 
thing other than the virus protein by 
ultracentrifugation. Dr. Spencer recog- 
nized this possibility and suggested 
that precursor macromolecules that 
could be activated into virus might 
have been present. This possibility has 
recently found support in the discovery 
of a noninfectious macromolecule sim- 
ilar to virus particles in tobacco leaves 
by Takihashi and Ishii. Bawden and 
Kassanis also differ with Spencer in 
their belief that virus can multiply at 
the expense of normal protein because 
they found the ratio of virus to other 
constituents was greatest in nitrogen- 
deficient tobacco. They also found 
that phosphorus increased both the 
growth of plants and concentration 
of virus in the sap. 

More data arc needed on the effect 
of mineral nutrients on virus diseases. 
There is fair agreement that vigorously 
growing plants are most subject to 
infection and injury particularly if 
cither nitrogen or balanced nutrients 
arc in excess supply. The lower specific 
activity of virus from nitrogen-defi- 
cient plants has been confirmed by M. 
Chessin, who found no difference in 
the size of virus particles from plants 
receiving nitrogen and those deprived 
of it. 

The principal effects of soil fer- 
tility on disease development are exem- 
plified by the diseases I have discussed. 
These particular diseases have been 
among the more thoroughly investi- 
gated and economically important 
ones. Many more plant diseases are 
known to be affected by soil condi- 
tions but little would be gained by 
discussing other individual cases. A 
brief summary of some of the general 
principles involved may be of value 
in understanding how various types of 


diseases arc influenced by soil fertility. 

Different fertilizer procedures arc to 
be employed for damping-off and root 
rot, the wilts, the galls and other over- 
growth disease, and the leaf mildews 
and rusts caused by obligate parasites. 

Damage from damping-off and root 
rot is avoided by promoting disease- 
escaping growth habits, such as rapid 
development of roots. The critically 
important nutrients for such growth 
are nitrogen and phosphorus; conse- 
quently properly balanced nitrate and 
phosphate fertilizers are beneficial. 

The wilt diseases arc most critically 
affected by the ratio of nitrogen and 
potassium. The gall and overgrowth 
diseases arc appreciably affected by 
the calcium-potassium ratios, partially 
modified by hydrogen-ion effects. Po- 
tassium and nitrogen are decisive fac- 
tors in influencing infection of leaves 
by obligate parasites. 

None of these treatments completely 
alters the inherent reaction of a plant 
to pathogens. Immune plants are not 
attacked regardless of the soil they arc 
grown in. Extremely susceptible vari- 
eties cannot be immunized by fertiliz- 
ing the soil. The greatest benefits from 
soil fertilizers have been observed on 
moderately susceptible to partially 
resistant varieties. Most horticultural 
varieties are in this latter class since 
they often escape infection or recover 
from disease. Proper fertilizers improve 
their opportunities to escape destruc- 
tion and produce a profitable crop. 

The concentration of properly bal- 
anced nutrients in the soil may influ- 
ence the .severity of di.sease. As growth 
is promoted by more liberal fertilizer 
applications, the damping-off and root 
rot diseases ^come less serious, but the 
galls, clubroot, and .scab diseases be- 
come more conspicuous and leaf rust 
and powdery mildew caused by obli- 
gate parasites flourish. If sufficient 
balanced fertilizers are applied to 
injure the plant and retard growth, 
damping-off and seed decay are more 
destructive and the obligate parasites 
may be less active. 

The balance of nutrients may be 



THE EFFECTS OF $OU FERTILITY 


more important than concentration of 
total fertilizer when plants are exposed 
to attack by parasites. A deficiency or 
surplus of any one element often pro- 
motes disease. 

All classes of diseases — from those 
caused by the facultative saprophytes 
that destroy stored fruits and vege- 
tables to those caused by the obligate 
parasites — are most severe when nitro- 
gen is overly abundant. Although it 
encourages infection of wheat roots 
by the fungi that cause the browning 
disease and take-all, its over-all effects 
may be beneficial if it promotes growth 
of new roots. If phosphorus, however, 
is not available to facilitate root de- 
velopment, the excess nitrogen is fatal 
to the root system of the plant. 

An excess of nitrogen promotes wilt 
diseases by providing better nourish- 
ment for the vascular parasites. Am- 
monium salts are usually more readily 
used by the parasite so they are more 
damaging than nitrates unless the lat- 
ter are reduced to nitrites, which are 
poisonous to plants. A deficiency of 
potassium, which automatically cre- 
ates an excess of nitrogen and carbo- 
hydrates in the plant, also increases 
wilting by most vascular parasites. 

A deficiency of potassium increases 
the severity of many diseases. Potash 
fertilizers have alleviated damage from 
more diseases than any other nutri- 
tional treatment. The importance of 
potassium has not been explained, 
but it is probably due to its ability to 
regulate chemical reactions in the cells 
of the plant. A deficiency of potassium 
under most circumstances implies an 
excess of nitrates and phosphorus; thin- 
ner cell walls in epidermal tissues; re- 
duced production of amino acids be- 
cause nitrate reduction is suppressed; 
accumulation of carbohydrates which 
cannot be synthesized into proteins; 
failure to produce new cells for want 
of essential amino acids for the proto- 
plasm; and slower growth of meriste- 
matic tissues that would permit re- 
placement of diseased tissues. 

These changes may facilitate pene- 
tration of the epidermis by plant 


parasites, increase their metabolism 
and growth in plant tissues, or promote 
destruction of the entire plant because 
it cannot develop new tissue to replace 
those lost by ravages of the pathogens. 

The only diseases that arc consist- 
ently suppressed by potassium deficien- 
cy are the galls and overgrowths that 
depend upon multiplication of cells. 
Most of the overgrowths increase con- 
spicuously as the potassium supply is 
increased while wilts and leaf diseases 
diminish. 

Soil reaction and organic matter 
coptent of soils must be considered as 
r^ulants of soil fertility because they 
affect the availability of the nutrient 
elements. Such materials as the phos- 
phates, calcium, iron, manganese, and 
boron are not available to plants in 
alkaline soils since they may be precipi- 
tated as insoluble salts. Applications of 
organic matter contribute nutrients, 
mastly nitrogen and potassium, but 
the immediate effect is to remove all 
surplus available nitrogen by combin- 
ing with it or by promoting growth of 
soil micro-organisms that assimilate ni- 
trogen to the point of temporarily de- 
picting the soil supply. The effect on 
diseases may be good or bad, depend- 
ing upon the type of disease and the 
stage of its development. 

Soil reaction and organic matter also 
influence the prevalence of plant path- 
ogens. Some pathogens prefer alkaline 
soils and others thrive in acid soils .so no 
general rule can be laid down. If soils 
are extremely acid or alkaline soil mi- 
cro-organisms may be eliminated. Or- 
ganic matter is essential for growth of 
most soil inhabitants irrespective of 
whether they are beneficial or destruc- 
tive. 

Many plant para.siies increa.se and 
are dis.seminated on organic matter, 
such as plant refuse, manures, and 
compost. The addition of organic mat- 
ter to soil, however, often .suppres.scs 
pathogens if they arc poor soil invad- 
ers. The organic matter stimulates 
growth of soil saprojihytes that deprive 
the less aggressive pathogens of min- 
eral nutrients or else they secrete toxic 



YEARiOOK OF AORICUITORE 1953 


114 

antibiotics. Only a few plant disease 
agents are affected appreciably, since 
most of them arc effective soil inhabi- 
tants and compete successfully with 
the saprophytic micro-organisms. 

Probably the greatest benefits de- 
rived from organic matter arc in soil 
stabilization. The humus from lignified 
tissues improves the physical structure 
of soil so it does not erode and will 
hold more moisture. Application of 
barnyard manure, straw, crop residues, 
or even carbohydrates such as starch 
and sugar stimulates the soil micro- 
organisms to growth so they often 
synthesize all available nitrates into 
their bodies. This removes surplus 
nitrates from the soil; later they are 
released gradually so that the plants 
have a more continuous supply of 
nitrogen; excessive concentrations, 
which predispose them to so many 
diseases, are avoided. 

This ability of organic matter to 
stabilize the soil solution and make 
the soil into a desirable medium for 
root growth fully justifies its use. 
The material may not have any 
unusual properties that would make 
plants resistant to all diseases. It can 
and docs increase the severity of some 
diseases, particularly when applied to 
soils at the wrong time, but it can 
also be used to advantage for other 
diseases. There Is no evidence that 
it encourages soil bacteria and fungi 
to produce antibiotics that arc taken 
into the plant and help immunize it. 
As a matter of fact, there is evidence 
that some antibiotics are inactivated 
when added to soil. 

No general rules can be laid down 
about fertilizing soils so as to avoid 
disease. Each disease must be con- 
sidered by itself. 

Any sound recommendation must be 
based on the type of soil, the avail- 
ability of essential nutrients in the 
soil, and the character of the disease 
agents that are most likely to strike 
plants in the area. Conspicuous soil 
deficiencies, particularly in potash, 
should be corrected. 

Every effort should be made to avoid 


surpluses of nitrogen that are not 
ne<^ed for steady and strong growth 
of the plant. Fertilizers may often 
be used to advantage in controlling 
acidity so the soil reaction will be 
favorable to the crop and unsuited 
to maximum development of the 
plant pathogens. 

George L. McNew is managing 
director of the Boyce Thompson Institute 
for Plant Research at Yonkers^ N. Y, 
Before taking that position in he was 
professor and head of the botany department 
at Iowa State College and manager of 
agricultural chemical research and develop- 
ment of the United States Rubber Company 
at Bethany^ Conn. Earlier he conducted 
research on vegetable crop diseases at the 
New York Agricultural Experiment Station^ 
Geneva, N Y., and on the fundamental 
nature of parasitism in bacterial plant 
pathogens at the Rockefeller Institute for 
Medical Resedrch at Princeton, A*. 



Diseased root of cabbage plant. 




How Fungicides 
Have Been 
Developed 

John C. Dunegan^ S. P. Doolittle 

How to keep school boys from pilfer- 
ing their grapes had been a problem 
of French peasants in the Mcdoc region 
for a long time. 

For the pilferers they found the 
answer, a poisonous-looking mixture of 
lime and copper sulfate that stuck 
tenaciously to the leaves when sprin- 
kled on vines near the roadsides. 

Pilfering was not the peasants’ only 
worry, for mildew — mildiou^ they called 
it — every year defoliated the vines in 
early fall. Professor P. M . A. Millardet, 


of the University of Bordeaux, was 
commissioned to study this disease 
recently introduced from America. 

One day, in 1882, as he walked 
through the Medoc countryside, he 
noticed that there was less mildew on 
the sprinkled vines. 

The professor put tw^o and two to- 
gether: The cure for pilfering might 
be the cure for the mildew. The next 
year he started tests that confirmed 
his guess. He and his colleague, U. 
Gayon, in 1885 published illustrations 
of treated and untreated vines. By 
1887 they claimed unqualified success. 

The mixture became famous as 
“Bouillie Bordelaise”“^bordeaux mix- 
ture — and was applied with fiber 
brooms or, later, with sprayers in 
vineyards ravaged by vine mildew and 
also in blighted fields of potatoes and 
tomatoes. 

Thus only 70-odd years ago chemi- 
cal weapons came into use to control 
blights, rusts, mildews, and fungus 
diseases that for countless centuries 


1 15 




YEARBOOK OP AGRICULTURE 1953 


1 16 

have robbed men of the fruits of their 
labor. 

Chemicals used to control fungus 
diseases are called fungicides, a word 
derived from the Latin Jungus^ a 
microscopic plant, and caedo^ “I kill.” 

Originally the term was restricted 
to any substance applied to higher 
living plants in active growth to kill 
parasitic fungi or prevent the develop- 
ment of fungus diseases without seri- 
ously injuring the host plant. The 
meaning has been further broadened 
and as currently used denotes any 
substance or mixture of substances 
used for controlling fungi present in 
any environment. Thus, materials used 
to prevent molds from destroying the 
insulation of electrical equipment or 
etching camera lenses are properly 
called fungicides. 

In agriculture, fungicides by and 
large are used to prevent infection, 
because once the fungus has caused 
any extensive alteration of the part at- 
tacked little is to be gained by killing 
the fungus. To do so will not repair the 
damage already done, although it 
may prevent the further spread of the 
fungus. 

The virtues of bordeaux mixture 
were soon demonstrated, but so were 
its faults. On certain plants the mix- 
ture produced injury. Attention was 
turned to other materials. In 1905 
A. B, Cord^ey introduced lime-sulfur 
solution, a mixture of calcium poly- 
sulfides formed by boiling sulfur and 
lime together in water. The material 
proved to be exceptionally efficient 
against the apple scab fungus and 
eliminated the russeting of the fruit 
that follow'cd the use of bordeaux 
mixture early in the season. 

Lime-sulfur s<5lution, however, 
proved to be too caustic to use on 
peaches. In 1907 W, M. Scott and 
T. W. Ayers introduced another sul- 
fur mixture they called “self-boiled 
lime-sulfur.” 7 'hcy made it by adding 
sulfur to stone lime slaking in water. 
Enough heat was produced by the 
slaking of the stone lime to cause a 
mild chemical reaction. The mixture 


turned out to be too mild to control 
the apple scab fungus, but it proved 
to be effective against the peach brown 
rot and scab fungi, the cause of two 
serious peach diseases. It did not injure 
peach trees and it made the commer- 
cial production of peaches practical in 
humid sections of the East. 

Bordeaux mixture proved to be 
good for most vegetable diseases and 
for such midseason apple diseases 
as blotch and bitter rot. With it and 
lime-sulfur solution for early use on 
apples and pears for scab control and 
self-boiled lime-sulfur for use on 
peaches, pathologists felt they were 
well equipped to prevent fungus 
diseases on fruits and vegetables. 

Indeed, from 1907 until about 1930, 
the three preparations were standard. 
Experiments were concerned largely 
with tests of the proper timing of 
sprays, the number of applications, 
and the amounts to be used, rather 
than with the development of new 
materials. 

But as time went on, people noticed 
that in seasons when the fungi did not 
develop extensively the yield from 
unsprayed tomato plants frequently 
was better than from those protected 
by the bordeaux mixture. Probably it 
was the result of physiological changes 
induced by the residue of copper and 
lime on the plants. Also, they found 
that the lime-sulfur solution affected 
the leaf and shoot growth early in the 
season and had an adverse effect on 
the setting of the fruit of certain apple 
varieties. Bordeaux mixture too fre- 
quently caused a russeting of the fruit, 
severe injury to the leaves, and a pre- 
mature defoliation. The main ob- 
jection to self-boiled lime-sulfur was 
the need of stone, or unslaked, lime, 
which was cumbersome to handle and 
had to be stored in sealed containers. 

Those defects led to the develop- 
ment of entirely new compounds as 
technicians began to explore the fun- 
gicidal properties of other inorganic 
compounds such as the fixed (rela- 
tively insoluble) copper compounds 
and the organic compounds. The 



HOW rUNOICIOiS NAVf iCEN DEVELOPED 


latter compounds exist in almost 
countless numbers. Some 25,000 have 
already been tested as possible fungi- 
cides and new compounds are con- 
stantly being synthesized in the re- 
search laboratories. 

Obviously, with such a multitude of 
compounds available, some orderly 
testing or screening procedure is 
necessary to expedite the big task. It 
would 1^ physically and financially 
impossible to test each compound by 
actual spraying tests in the fields. 

In these screening tests, spores of 
certain disease-producing fungi are 
suspended in solutions or suspensions 
of the chemical. They are removed at 
stated intervals, and their viability 
is tested. An alternate method is to 
expose spore suspensions of the test 
organisms to the chemical on glass 
slides and later determine whether 
the spores have been killed by contact 
with the chemical. 

The test can be further improved by 
determining the effect of environment 
upon the dried deposits of the chemi- 
cals exposed on glass slides outdoors 
for varying periods. Tests are made 
at fixed intervals, frequently at the 
end of 7, 14, and 2i days and are 
compared with the effects of freshly 
prepared residues on the fungus 
spores. The tests tell whether the 
exposure outdoors has affected the 
fungicidal activity of the chemical and 
have eliminated unstable compounds 
that appear promising when freshly 
prepared. Other phases of the screen- 
ing procedure deal with the physical 
properties (particle size, solubility, 
tenacity) of the material. As the result 
of these various laboratory studies it is 
now possible to compare the effective- 
ness of various compounds at different 
dosage rates and even to determine 
the portion of the complex organic 
molecule that actually kills the fungus. 

Special types of spraying and dusting 
equipment have been devised for 
laboratory use that deliver known 
amounts of material with great pre- 
cision. These machines make it pos- 
sible to duplicate in the laboratory 


II7 

or greenhouse many of the factors 
that occur in the normal use of the 
compound and are an important part 
of tlie screening procedure. 

Compounds that have passed through 
the various screening tests must still sur- 
mount the final barrier of actual field 
usage. They must control, the fungus 
diseases, must be compatible with com- 
pounds added for the control of insects, 
and must not, either alone or in com- 
bination, produce injury to the treated 
plants. Furthermore, the new com- 
pounds must not be so toxic that their 
widespread use presents dangerous haz- 
ards either to the user or the consumer 
and finally their price must be such 
that they can compete with other com- 
pounds currently in use. 

The development of a new compound 
therefore is a tedious and expensive 
process. On the average this develop- 
ment involves an investment between 
250,000 and 350,000 dollars and may 
even exceed a million dollars. 

“Many are tested but few are cho- 
sen,” to modify an old adage, aptly 
applies to the development of new 
fungicide materials. Less than o.i per- 
cent of the many thousands of com- 
pounds tested are in general use. 

Singe 1 930, many new materials for 
use on fruits and vegetables have been 
developed. They include: 

Copper (silicates, basic sulfates, chlo- 
rides, oxides, phosphate, and certain 
copper organic combinations). 

Mercury (phenyl derivatives — ^lac- 
tates, acetates, formamides). 

Dithiocarbamates (thiuram disul- 
fides — tetramethyl and morpholine; 
metallic methyl derivatives — ^iron, zinc, 
and lead; metallic ethylene deriva- 
tives — sodium, zinc, manganese). 

Chlorinated quinones (tetrachloro- 
quinone). 

Chlorinated naphthoquinones (di- 
chloronaphthoquinone) . 

Quinolinolates (copper and zinc 
compounds). 

Quaternary compounds (quinoline, 
isoquinoline). 

Glyoxalidines. 



YEAKBOOK OF AOBICUITUBB 1953 


Il 8 

Nitrated phenols (dinitro com- 
pounds). 

Chlorinated phenols (pentachloro- 
phenates). 

Thalamides (N-trichloromethylthio 
tctrahydrophthalimidc) . 

Chromium compounds (complex 
double salts with other metals). 

The control obtained with some of 
these new compounds is outstanding. 
The effect of ferric dimethyl dithio- 
carbamate (ferbam) on cedar-apple 
rust is an example. Before it was in- 
troduced, sulfur gave only indifferent 
control of this disease, but this iron 
dithiocarbamate has practically solved 
the problem of cedar-apple rust for the 
commercial apple grow-er. The same 
compound has eliminated the spray- 
injury problem in the production of 
high-quality pears in the Pacific North- 
west. 

Many of the new compounds have 
proved satisfactory for the control of 
plant diseases in the Tropics. An im- 
mense potential demand exists for the 
products in the Tropics, but their wide- 
spread use is limited by the relatively 
low purchasing power of many of the 
tropical farmers. Strangely enough, no 
organic compound has been found to 
replace bordeaux mixture for the con- 
trol of the very destructive Sigatoka 
disease of bananas. So intensive is the 
attack of the fungus causing this leaf 
disease, banp.na plantations must be 
sprayed 1 5 to 17 times a year. Approx- 
imately 45 million pounds of copper 
sulfate are required each year to pro- 
tect the 1 30 thousand acres of bananas 
growing in tropical America. 

While these new compounds have 
given better control of the fungi, they 
have been less satisfactory against 
plant diseases caused by bacteria. 
Those diseases present a particularly 
difficult problem, for the bacteria are 
present in such enormous numbers 
that it is difficult to kill all of them 
by ordinary procedures. 

Current research for the control of 
bacterial diseases of plants is centered 
on ■ the possible use of antibiotics. 
These materials, derived from the 


fungi themselves, have given brilliant 
results in the field of human pathology, 
and research workers in various parts 
of the world are trying to determine 
if they are just as effective in their 
action against the bacteria that cause 
plant disease. The investigations are 
still in the preliminary stages, but 
even so it is evident that some anti- 
biotics are absorbed by the plant and 
transported into the newly developing 
leaves and stems. This absorption 
suggests the possibility of protecting 
the entire plant against invasion by 
disease-producing bacteria. 

Those tests are not limited to the 
use of antibiotics. The action of 
chemicals injected into the stem of 
living plants or applied as solutions 
to the soil around the roots also is 
being investigated as a possible mode 
of attack against virus diseases of 
plants — a group of important and 
destructive maladies that arc difficult 
to control. 

The improvements in fungicide ma- 
terials would be of little value had 
they not been accompanied by an 
analogous development in the ma- 
chines used for applying them. Bor- 
deaux mixture was firet applied by 
sprinkling it on the plants with 
brooms or brushes. This tedious pro- 
cedure soon led to the development 
of simple, hand-operated hydraulic 
pumps. The spray material was 
forced, under pressure from the pump, 
through a jet-lipped rod and deposited 
on the various plant parts as the 
operator moved the spray rod among 
the plants. The bucket pump, the 
knapsack sprayer, the barrel-type 
sprayer, and the one- or two-cylinder 
hydraulic pumps (either hand-oper- 
ated or driven by a chain-drive 
sprocket connected to one wheel of 
the wagon carrying the pump and 
spray supply in a barrel) are repre- 
sentative of the pioneer types of 
sprayipg equipment. 

The gallons-per-minute output of 
all these rigs was low, operating them 
was hard work, and the acreage they 
could cover was limited. Nevertheless 



HOW FUMOICIDES HA 

the benefit obtained, particularly when 
diseases required only one or two 
applications, was striking, indeed. 
Sprayed plants remained healthy and 
produced good crops, while the un- 
sprayed plantings had to be aban- 
doned because of disease. 

Just as the development of some of 
the new organic fungicides evolved 
from compounds used in the manu- 
facture of automobile tires, so did 
the development of efficient spray 
machinery evolve with the develop- 
ment of the gasoline engine for auto- 
mobiles. Indeed, modern control of 
plant diseases on a large scale became 
feasible just as soon as the gasoline 
engine was substituted for manpower. 
Pumps were redesigned to operate at 
higher speeds, develop more pressure, 
and have a higher gallons-per-minute 
output than was possible with manu- 
ally operated machines. The gasoline 
engine also permitted the introduction 
of a power-driven agitator to keep the 
spray materials thoroughly in sus- 
pension, a feature that was inade- 
quately provided for in the manually 
operated machines. 

At first there were rather cumber- 
some rigs powered by heavy, one-cyl- 
inder, two-cycle engines of low horse- 
power. The machines gradually were 
more refined. Wooden wheels on the 
truck were supplanted by steel wheels, 
which in turn were replaced by rubber- 
tired wheels. The heavy, one-cylinder 
engines were replaced by two-cylinder 
and four-cylinder engines, which de- 
veloped increasing horsepower. Pump 
pressures increased from loo to 600 
pounds pressure to the square inch, 
and the output in gallons a minute 
from 2 to 3 to as high as 40 or 50, 
with the present-day high-capacity 
machines. 

The increase in pump capacity 
necessitated larger tanks, and machines 
with 400- and 6oo-gallon tanks be- 
came standard equipment. The limit 
on tank size appears to be 600 gallons 
— ^because the weight of that much 
water, 4,800 pounds, is about the 
maximum that can be drawn over 


Ve lEEN DEVELOPED II9 

soft ground. Some large commercial 
orchards use pumps in a central 
station and a network of pipes with 
suitable outlets or risers spaced through 
the planting. It would seem to be an 
ideal solution, but large investment in 
pipes and machinery is required, 
pipes break in winter, and the spray 
chemicals corrode equipment all the 
time. Stationary spray plants are 
practical only in the very large com- 
mercial operations. They probably 
reached their maximum use in the 
banana plantings in Central America. 

Power-driven spray machines also 
have been develops for use with 
vegetable crops. The problem there is 
to cover each plant in the rows 
adequately with fungicides. That is 
done by a series of nozzles on hori- 
zontal booms arranged so that the 
.spray envelopes the plants. In some 
machines the booms simultaneously 
spray all plants in a swath 40 feet 
wide. Their development has made 
possible the protection of extensive 
acreages of row crops. 

Likewise, for use in vineyards and 
plantings of brambles on trellises, 
spray machines have been devised 
with vertical instead of horizontal 
booms. Some machines are designed 
to cover both sides of an individual 
row. Others spray on both sides of 
the machine as it moves up the rows. 

The motor-driven spray rigs re- 
quired two or three men. Wartime 
manpower shortages and high wages 
meant that faster and cheaper ways 
of applying sprays had to be found. 
First in citrus orchards and then in 
peach, apple, and pear orchards a 
new machine was put into operation. 
It was called a speed sprayer. It had 
a 6oo-gallon tank, an engine of 80 
horsepower or more, conventional 
pumps, and spray nozzles arranged 
as adjustable arcs on both sides of the 
machine. Men were not needed to 
direct the spray to various parts of 
the tree. The spray, as it was forced 
from the nozzles, was caught in the 
blast of air from a rapidly revolving 
propeller and blown into the trees. 



120 


YEARBOOK OF AGRICULTURE 1953 


The machine was so heavy that it 
could be pulled only by a tractor, but 
the controls were so arranged that 
the driver could spray the trees. 

Many years earlier, the use of fine 
powders or dusts had cut the cost of 
operation and reduced the amount of 
water needed. But the method has 
faults: The deposits of fungicidal dusts 
apparently do not adhere quite so 
well as "Spray deposits. Because of the 
lighter weight of the equipment and 
the speed of application, however, 
dusting is used often with success. 

All in all, the trend in 1953 was to 
combine the advantages of several 
methods. One was to increase the ad- 
herence and effectiveness of dusts by- 
injecting water and adhesives into the 
dust stream under relatively low pres- 
sure (70 pounds the square inch). 
Another was to use machines equipped 
with highly efficient fans or specially 
designed turbines, which develop high- 
velocity air blasts carrying either dust 
or liquid particles to all parts of tlie 
plants to be protected. Sometimes these 
modem efficient blowers were available 
as units for the modernization of older 
equipment at a minimum cost. Still 
another procedure was to use sprays in 
concentrated form. If, say, 20 gallons 
of dilute spray w'ere needed to protect 
a given area, the same amount of 
fungicide was applied in 5 gallons of 
water — a 4 times (or 4.x) concentration. 
Such concentrated sprays reduce the 
amount of water needed, but to gel 
proper protection the rate of delivery 
of the pumps and nozzles and the for- 
ward movement of the spray machine 
must be in careful adjustment. 

John C. Dunegan is a pathologist in 
charge oj investigations oj jungus and bac- 
terial diseases oj deciduous fruit trees in the 
division of fruit and nut crops and diseases^ 
Bureau of Plant Industry^ Soils, and Agri- 
cultural Engineering. 

S. P. Doolittle is a pathologist in 
charge of investigations of diseases of 
tomatoes, peppery cucurbits, and certain 
other vegetable crops in the division of vege- 
table crops and diseases in the same bureau. 


Using Chemicals 
To Combat 
Root Diseases 


Jesse R. Christie 

Many of the myriad of organisms 
that inhabit the soil subsist on living 
plants. They may injure the roots and 
other underground parts of the plants 
and interfere seriously with growth. 
The damage they cause is likely to 
become more serious the longer the 
land is in cultivation, especially if the 
same crops are grown repeatedly in 
the same place. 

Their control is a problem that we 
have not yet solved entirely. Suitable 
crop rotations and the elimination of 
weeds that may serve as the hosts for 
the disease organisms, the planting of 
resistant varieties, and other cultural 
practices help reduce losses of crops, 
but often they are not enough. 

Control by the use of chemicals has 
met with considerable success, espe- 
cially with vegetables. Volatile liquids 
that function as soil fumigants have 
come into use extensively since 1945. 
Thousands of acres are fumigat^ 
annually, and the acreage is increas- 
ing. Apparently growers have found 
the procedure profitable. Soil fumi- 
gation is costly, but the increased yield 
^lerwards may have a cash value 
very much more than the investment. 
Sometimes fumigation makes the dif- 
ference between a profitable crop and 
none at all. 

To the bacteria, fungi, nematodes, 
and insects, to which most of the 
noxious organisms belong, it will be 
practical for us to add weed seed. 
Soil fumigation has been most sue* 



121 


USING CHEMICALS TO COMBAT lOOT DISEASES 


cessful in controlling nematodes; some 
of the fumigants kill other pests; some 
other methods are as effective as fumi- 
gation and less expensive. When one 
has to fumigate for nematodes, how- 
ever, the control at the same time of 
insects and other pests is important. 

Soil fumigants are sold under vari- 
ous trade names, but only four dis- 
tinctly different kinds are in general 
use. They arc composed of (or have 
as their active ingredient) methyl 
bromide (bromomethane), chloropic- 
rin (trichloronitromethane), dichloro- 
propenc (1,3-dichloropropcne), and 
ethylene dibromide, which is 1,2-di- 
bromoethane. 

For seedbeds, soil fumigation or any 
other chemical treatment should be 
effective against a variety of organisms. 

Damping-off is notoriously serious 
in seedbeds, and failure to control it 
or reduce losses from it is a serious 
defect of a treatment. 

A soil treatment that will kill weed 
seed and thereby eliminate the cost of 
weeding may effect savings that more 
than pay the cost of even an expensive 
chemical. 

Root knot is an ever-present prob- 
lem in the South. Its control in seed- 
beds is especially important. Other 
nematode problems may be equally 
serious in some regions — ^for example, 
control of the stubby root nematode 
(species of Trichodorus) in the seedbeds 
of certain celery-growing regions in 
Florida. Of the fumigants in general 
use, methyl bromide and chloropicrin 
come nearest to fulfilling those re- 
quirements. 

Methyl bromide, a highly volatile 
compound, has a boiling point of 
about 40® F. In order to inject it into 
the soil, in the manner that most 
fumigants are applied, it must be 
mixed with a diluent having a higher 
boiling point. Such mixtures have 
been in use for some years, but with 
them it is hard to keep the gas in the 
soil long enough to obtain the desired 
results. 

A newer method makes it possible 
to utilize, to a much greater degree, 


the killing power of the chemical. 
Undiluted methyl bromide is evap- 
orated in shallow trays placed on the 
surface of the soil under a gastight 
cover. The cover is held up a few 
inches by supports, so there is a shal- 
low air space between it and the sur- 
face of the soil. The edges of the cover 
are buried. The rate for most pur- 
poses is 1 pound to 100 square feet, 
but for controlling the more resistant 
fungi, rates up to 4 pounds to 100 
square feet are recommended. The 
cover should remain in place for 48 
hours. Usually seed can be planted 2 
to 3 days after the cover is removed. 
Penetration of the gas into the soil is 
surprisingly good. Under favorable 
conditions, the sterilizing effect ex- 
tends to a depth of 1 2 inches or more. 
Most or all of the insects are killed. 
Miscellaneous soil nematodes are 
nearly eradicated. Control of root 
knot is usually good, although not 
always complete. All living plants are 
killed, including the most tenacious 
of the grasses and ail but a few of the 
more resistant weed seed. On the 
basis of some tests, the control of fungi 
seems satisfactory. The main disad- 
vantage of the method is cost of the 
cover and equipment needed to treat 
large seedbeds and the amount of 
time and labor involved in moving 
equipment from place to place. 

Chloropicrin has been on the mar- 
ket longer than any other soil fumi- 
gant in general use. It is expensive 
and disagreeable to handle, but many 
vegetable growers prefer it for fumi- 
gating seedbeds. Properly applied, it 
will kill insects, nematodes, most weed 
seed, and all except a few of the more 
resistant fungi. Chloropicrin is in- 
jected into the soil in the same manner 
as most other fumigants. Injection 
points — or continuous streams if power 
equipment is used — should be spaced 
10 inches apart. The recommended 
ratQ of application is 2 cubic centi- 
meters to 3 cubic centimeters (about 
one-half teaspoonful) at each point. 

A water seal must be applied im- 
mediately. For best results, the area 



122 


YEAtSOOK OF AOIICULTUEB 1953 


should be covered with burlap sacks, 
canvas, newspaper, or something like 
them, which, in turn, should be 
sprinlded with water. The cover, when 
it is used, may be removed after 4 or 
5 days. The soil is allowed to aerate. 
Seed must not be planted until every 
trace of the gas has disappeared, which 
usually takes 8 to 12 days; in wet, cool 
weather it may take longer. 

Ghioropicrin forms a gas that is ex- 
tremely toxic to plants, in both the 
soil and the air. Under certain weather 
conditions, a blanket of gas may col- 
lect over a fumigated bed near the 
ground, then drift slowly over a near- 
by area, and cause severe injury to the 
plants growing there, especially at 
night when foliage is wet with dew. 
That does not alw'ays happen, but it 
is a risk that should be remembered 
when one fumigates seedbeds near 
growing crops. 

Chlorobromopropene, commonly 
called GBP, is a promising new chem- 
ical for the treatment of sccdlxids. 
When emulsified with water and ap- 
plied as a drench it has given good 
control of nematodes, fungi, and weeds 
in tests conducted on the sandy soils 
of central Florida. Equipment that 
has a small gasoline-driven pump, 
which mixes the chemical with water 
and delivers the mixture through a 
plastic hose to a sprinkling nozzle, 
makes application quite easy. 

Mixtures containing dichloropro- 
pene or those containing ethylene 
dibromide are also used for fumigating 
seedbeds. They give good control of 
nematodes and soil insects but neither 
has much effect on weed seed or fungi. 

The practice in some regions is to 
prepare beds that are 6 inches or more 
higher than the walkways between. 
When methyl bromide, chloropicrin, 
or GBP are used, the beds are first 
prepared and then treated. The walk- 
ways are not treated. When ethylene 
dibromide or dichloropropene ^arc 
used, a common practice is to fumi- 
gate the entire area, and prepare the 
beds afterward. The rates usually rec- 
ommended are higher than those for 


ordinary field applications: 35 to 45 
gallons an acre of a dichloropropene 
fumigant or 30 to 40 gallons of 40 
percent ethylene dibromide. 

Ethylene dibromide and mixtures 
containing dichloropropene are gener- 
ally used for fumigating fields. Both 
are excellent nematocides and good 
insecticides, but at ordinary rates 
neither has much value as a fungicide 
or herbicide. Ethylene dibromide is 
more effective against wireworms. 
Either fumigant substantially reduces 
the population of the mole crickets in 
Florida by killing the insects or else 
driving them to the surface, where 
they are eaten by crows and blackbirds. 

All dichloropropene fumigants have 
about the same strength — the same 
percentage of active ingredients. Ethyl- 
ene dibromide fumigants are mixtures 
of ethylene dibromide and a diluent, 
usually naphtha. The diluent adds 
nothing to the efficacy. It is used 
merely to increase volume and thereby 
facilitate accurate and uniform appli- 
cation. The different trade-marked 
fumigants of this kind differ in the 
amount of ethylene dibromide they 
contain. The two most commonly used 
for field-scale applications contain 
cither 41 percent or 83 percent of 
ethylene dibromide by weight. The 
83 percent fumigants usually are 
diluted by the dealer or by the farmer 
before applying. If proper equipment 
is available, they may be applied 
without diluting. 

The entire area of a field may be 
fumigated. The procedure is called 
solid, or broadcast, application. Or 
the fumigant may be applied only in 
the rows or hills where the plants arc 
to grow. For solid application, stand- 
ard practice is to space injections 12 
inches apart. The recommended dos- 
age for a dichloropropene fumigant 
is 20 to 25 gallons an acre. For a 41- 
percent ethylene dibromide fumigant 
it is 15 to 20 gallons an acre. For row 
fumigation a single stream is applied 
along each row. The usual rate for 
either of the materials is about 2 cubic 
centimeters (about one-half teaspoon- 



USING CHEMICALS TO COMSAT ROOT DISEASES 


ful) per linear foot of row or i gallon 
to 1,900 feet of row. If rows are 3 feet 
apart, an acre will require about 8 gal- 
lons. Row fumigation has given satis- 
factory control of root knot on toma- 
toes and some other crops. 

A few tomato growers have adopted 
the practice of applying two streams, 
12 inches apart, along each planting 
row, but no one seems to have demon- 
strated that applying two streams is 
very much more effective than apply- 
ing about the same amount of fumigant 
in one stream. Good control of root 
knot on watermelons has been ob- 
tained by making a single injection in 
each hill. 

Row or hill fumigation wUl prove 
effective, no doubt, for controlling 
many — but not all — kinds of nema- 
todes. In experiments conducted in 
Florida for controlling the stubby root 
nematode on sweet corn, row fumiga- 
tion failed completely. Those nema- 
todes move into the fumigated area 
from the surrounding unfumigated 
soil so quickly that the plants arc given 
only very brief protection. 

Soil fumigation has been in use long 
enough and has been tested extensively 
enough to demonstrate that it is an 
effective and useful method, but re- 
quirements for success arc somewhat 
exacting and the factors influencing 
efficacy arc numerous and not fully 
understood. Failures may occur for 
many reasons, among them faulty 
diagnosis (attempting to correct trou- 
bles not caused by organisms that fumi- 
gation will control), faulty application, 
and insufficient aeration of the soil 
between application and planting. 

In places .where crop failures arc not 
caused by organisms that fumigants 
will control but occur for some other 
reason, soil fumigation is a waste of 
time and money. It is not advisable to 
fumigate on general principles. Diag- 
nosis should precede treatment. Often, 
however, determining the cause of crop 
failures is more easily said than done. 
When the cause of the trouble is in 
doubt, a wise procedure is to make trial 
applications on a small scale. 


123 

Fumigants to be successful must be 
properly applied. The requirements 
are not unduly difficult to fulfill and 
the reasons for them are easy to under- 
stand, but more failures have resulted 
from faulty application than from any 
other cause. 

The land should be thoroughly and 
properly prepared. The soil should be 
reasonably free from lumps and clods 
and should be moderately, not exces- 
sively, loose. Very light, sandy soils 
should be compact. When seedb^s are 
prepared with a rotary tiller, a few 
days should be allowed for the soil to 
settle, except if methyl bromide is to be 
evaporated under a cover. 

The soil should Ije moist but not wet. 
When the soil is even moderately dry, 
die efficacy of fumigants in killing most 
organisms is somewhat reduced, and 
ample moisture is especially important 
for effective control of weeds and cer- 
tain fungi. The nearer a weed seed Is 
to germination, the easier it is to kill, 
and fumigants are more likely to kill 
the sclerotia produced by some fiingi 
if those structures have been wet for a 
few days. 

The soil should be warm. Some fumi- 
gants are more effective than others at 
low temperatures, but generally the 
temperature of the soil should be at 
least 50® F. and preferably 60® or above. 

The fumigant should be injected at a 
uniform depth. If the ground is un- 
even, the cliisels of a power applicator 
will inject the fumigant too near the 
surface or even on the surface when 
they pass over low spots. The proper 
depth varies with conditions and the 
pests to be controlled. When chemicals 
are injected into the soil, organisms lo- 
cated near the surf£^:e are often not 
killed. This is an inherent weakness of 
soil fumigation regardless of the pests 
involved, and it is especially serious 
when attempting to control weeds or 
damping-off in seedbeds. 

In order to overcome this difficulty 
by increasing the concentration of gas 
near the surface, shallow application 
is recommended, with injection 3 or 4 
inches deep. For field application un- 



YEARBOOK OP AGRICULTURE 1953 


124 

der most conditions, 6 inches is usually 
recommended. In the sandy soils of 
Florida, roots that grow in the upper 
2 or 3 inches escape injury by nema* 
todes to a marked degree. Celery 
plants, growing on land that is heavily 
infested with the stubby root nema- 
tode, may produce a thick mat of more 
or less healthy roots in the upper 3 
inches of soil, although all the roots 
deeper than that are completely de- 
stroyed. The same is true, though to a 
somewhat lesser degree, for plants in- 
jured by the root knot nematodes. 
Hence, for field applications in Flor- 
ida, fumigants should be deeply in- 
jected, never less than 6 inches deep 
and preferably 7 or 8 inches. 

Holes or furrows made by the appli- 
cator should be promptly and firmly 
filled. If loosely filled, the gas, instead 
of diffusing into the surrounding soil, 
will escape upward into the air. 

Fumigants arc toxic to plants and 
should have diffused out of the soil 
before a crop is planted. Otherwise the 
plants may be stunted. The time re- 
quired for adequate aeration of the 
soil varies greatly and is influenced 
by many factors, including the kind 
of soil, the condition of the soil (espe- 
cially temperature and water content), 
the fumigant used, the rate of applica- 
tion, weather conditions following ap- 
plication, and the crop planted. Chlo- 
ropicrin, being extremely toxic, must 
be followed by thorough aeration. 
Some kinds of plants will tolerate low 
concentrations of ethylene dibromide 
without being seriously injured. The 
toxicity of dichloropropcne mixtures 
is intermediate between those two 
extremes, but, to avoid all danger of 
injury, aeration must be fairly thor- 
ough. Methyl bromide is highly toxic, 
but the gas leaves the soil so quickly 
that aeration for 2 or 3 days is usually 
adequate. 

The usual recommendation for di- 
chloropropene fumigants is to allow 
I week for every 10 gallons per acre. 
For, 41 percent ethylene dibromide, 
one should allow similarly varied but 
slightly shorter intervals. For most con- 


ditions, this rule allows a reasonable 
margin of safety, but there are excep- 
tions. If the soil is wet or cool and if 
the content of organic matter is high, 
those intervals may not be long enough. 
On the other hand, if the soil is light 
and the conditions are hot and dry, 
shorter intervals may be enough. 

The question has been rais^ as to 
the possibility of injury to the soil by 
use of soil fumigants and whether 
their continued application will re- 
sult in chemical, physical, or biological 
changes that may have a deleterious 
effect on the growth of plants. 

Three possibilities have been sug- 
gested: That the continued applica- 
tion of fumigants may eventually re- 
sult in the accumulation of toxic res- 
idues; that the chemicals will kill 
beneficial organisms and thereby se- 
riously interfere with the normal bio- 
chemical changes that occur in the 
soil; and that after land has been once 
fumigated the treatment must be re- 
peat^ each year, otherwise injurious 
organisms may become more serious 
than would have been the case had 
the land never been treated. 

Experiments have not been numer- 
ous enough or of sufficient duration 
to provide a final answer to the ques- 
tion of toxic residues, but results 
indicate that, with the fumigants now 
in use, we have litde to fear on this 
score. Very definitely, fumigation kills 
beneficial organisms, but most of them 
seem able to reestablish themselves 
rather quickly. The effect of fumiga- 
tion on the nonparasitic organisms of 
the soil generally is more transient 
than is its effect on the parasites. 
Obligate parasites, such ^s many of 
the nematodes, cannot reproduce ex- 
cept on a suitable host plant, and, 
although fumigation rarely extermi- 
nates them, at least one crop can be 
grown before their numbers increase 
sufficiently to cause serious damage. 

The conversion of nitrogen from an 
ammoniacal to a nitrate form is a 
biol^ical process. Destruction of ni- 
trifying bacteria by fumigation may 
retard the process and result in an 



USING CHEMICALS TO COMBAT BOOT DISEASES 


accumulation of nitrogen in the form 
of ammonia. Different plants differ 
in their ability to utilize nitrogen in 
this form. Tomatoes are reported as 
able to absorb ammoniacal nitrogen 
readily if the soil is neutral but not if 
it is acid. Hence it appears that the 
extent to which fumigation may 
possibly have an adverse effect on 
crops by reducing the nitrifying bac« 
teria of the soil depends on at least 
two factors — the kind of crop and the 
condition of the soil. 1 know of no 
instance in which fumigation has 
caused an accumulation of ammonia- 
cal nitrogen sufficient to interfere 
seriously with the successful growing 
of any crop. 

Fear that soil fumigation, if once 
begun but not continued, will be fol- 
lowed by an abnormal build-up of 
noxious organisms seems to have 
had its origin in results of experiments 
with the sugar beet nematode. Where 
land is heavily infested with that pest, 
the crop of sugar beets on fumigated 
areas may be very good, but the crop 
on unfumigated areas may be a failure. 
When sugar beets are grown on this 
same land the following year without 
fumigation, the situation may be re- 
versed and the crop may be more 
severely damaged on the areas that 
were fumigated the previous year 
than on those where no fumigant was 
applied. 

A suggested explanation is that dur- 
ing the first season on the unfumigated 
areas a huge number of nematode 
larvae hatched, attacked the roots of 
the plants, destroyed them, and, in so 
doing, eliminated their own food 
supply and thereby destroyed them- 
selves. On the fumigated areas the 
plants made an abundance of roots, 
so that the residual population of 
nematodes not killed by the treatment 
could build up rapidly. By the end 
of the season more cysts had been 
produced in the fumigated than in 
the unfumigated areas. If this is the 
correct explanation, these results are 
scarcely an indictment of soil fumi- 
gation. That treatment of the soil 


125 

with chemicals may permit popula- 
tions of plant parasitic nematodes to 
build up by destroying their natural 
enemies is a possibility. Not a great 
deal is known about these natural 
enemies and virtually nothing is 
known about the extent to which 
they hold the parasites in check. 

Research laboratories are active as 
never before in a search to find new 
and more effective chemicals for con- 
trolling pests of all kinds and those 
that inhabit the soil are not being 
slighted. Compounds under test in 
different localities show promise of 
being useful additions to the ones in 
use, especially for controlling fungi 
both in seedbeds and in the field. 
Some can be applied to the soil in 
powdered or granular form. Others 
can be mixed with water and applied 
as a drench, a procedure that may 
have advantages for certain purposes. 
Chemicals that will kill some or- 
ganisms and not others and can be 
applied around the roots of living 
plants seem a possibility. 

Root diseases are so serious and the 
need of better methods for controlling 
them is so pressing that new chemicals 
arc certain to come into use either 
with or without the sanction of experi- 
ment stations. While the purpo.se of 
such chemicals is to bring about 
desirable changes in the biology of 
the soil they may at the same time 
have other and undesirable effects. 
What these effects are, how serious 
they may become, and how they can 
be avoided or minimized are ques- 
tions that provide a research problem 
for the future. 

Jesse R. Christie, c native of New 
Hampshire^ is a nematologist in the Bureau 
of Plant Industry, Soils, and Af^ricultural 
Engineering, wluVh he joined in ig22. He is 
now stationed at the Central Florida Experi- 
ment Station, Sanford, Fla., and is in charge 
of the Bureau^ s nematode investigations in 
the Southeastern States. He holds degrees 
from the University of Kentucky, the Uni- 
versity of Illinois, and George Washington 
University, 



126 


YEARBOOK OF AGRICULTURE IBS3 


Fumigation 
of Soil in 
Hawaii 

Walter Carter 

As agricultural soils become older 
and cultivation continuous and more 
intensive, some soil amendment often 
is needed to offset the unfavorable 
effects on plants of the growing com- 
plex of pathogenic soil organisms or 
little known nutritional factors. 

If that can be achieved by adding 
large amounts of organic matter — such 
as green-manure crops — or if crop 
rotations arc established, the need for 
soil amendment is not so great as in 
areas where similar methods are not 
used. 

Some crops, howwer, cannot be 
grown successfully except occasionally 
in a long rotation. In many places in 
Great Britain, potatoes can be grown 
on the same land only one year in 
seven. In Utah, sugar beets require 
a 4-year or a 5-year rotation with 
other crops. Nematodes are the limit- 
ing factor. In tropical or subtropical 
areas, where active organic matter 
decomposes rapidly, the need for a 
soil amender is acute. 

Hawaii is no exception. Truck crops, 
particularly those that are susceptible 
to nematodes, cannot be grown profit- 
ably in succession on the same soils 
without the use of fumigants or other 
control methods. Pineapples have 
been grown in Hawaii for more than 
40 years on the same land without the 
addition of organic matter other than 
the residues of the previous crop, and 
the decline of productivity before 
fumigation became an established 


practice had been noted with increas- 
ing concern. One notable exception 
is a plantation where grass is grown 
for 2 years between pineapple plant- 
ings. 

An early attempt at soil amendment 
by fumigation in Hawaii in 1926 was 
directed primarily against insects and 
nematodes in sugarcane soil. A still 
earlier study, in 1910, was concerned 
with molasses as a fertilizer for sugar- 
cane. Fumigants were used in those 
experiments. The effect of fumigation 
with carbon bisulfide on nitrifying 
organisms was recognized as signifi- 
cantly affecting the availability of 
nutrients to the plant. The chemical 
did not destroy the micro-organisms 
but caused a reproportioning of them. 
The term is significant: It is not con- 
sidered practical to eradicate a micro- 
organism, but its position relative to 
that of the other organisms can be 
changed. 

Usually soil amendment by fumi- 
gation in Hawaii and elsewhere has 
been approached from the standpoint 
of control of nematodes and soil 
insects. As early as 1931, however, 
stimulation of the growth of pine- 
apples was recognized as being the 
result of partial soil sterilization. In 
1933 incrcjiLsed yields were recorded as 
having been obtained despite damage 
by nematodes. 

The first approach to the current 
viewpoint on soil fumigation in Hawaii 
was by the late Maxwell O. Johnson 
in experiments begun in 1927. He 
got striking increases in plant growth 
and yields of pineapples by the use of 
chloropicrin — tear gas. In his first 
experiments he applied this liquid to 
pineapple fields by means of a Ver- 
morel injector, a French device origi- 
nally used for the injection of carbon 
bisulfide into soil and stored grain. 

The first effect of the treatment was 
to produce a dark-green growth of 
the plant. Sometimes the fruit was 
larger. We now know that this was 
due, at least partly, to the killing of 
the nitrifying organisms in the soil by 
the chloi-opicrin. That meant that the 



FUMIGATION OF SOIL IN HAWAI 


plant used ammonium nitrogen rather 
than nitrate nitrogen. The pineapple 
plant fortunately is well adapted to 
ammonium nitrogen nutrition. John- 
son patented the use of chloropicrin as 
a soil fumigant in U. S. Patent No. 
1,983,546, which makes numerous 
claims, all of them concerned with 
plant stimulation. The killing by 
chloropicrin of such organisms as 
nematodes was known previously, at 
least academically, and it was therefore 
not included among the allowed claims. 

Chloropicrin has disadvantages. It is 
an extremely pungent and tear-making 
gas. It has always been relatively ex- 
pensive, so that its field-scale use is 
limited, especially as soil cover with 
water seals or with more or less im- 
permeable papers was essential to best 
results. Furthermore, at the lime John- 
son first used chloropicrin in Hawaiian 
pineapple soils, the favorable response 
to fumigation, so generally experienced 
now, was not consistent. Many applica- 
tions failed to give economic returns. 

The whole question of the field-scale 
use of the fumigants was completely 
changed by the discovery in 1940 that 
a mixture of 1,2-dichloropropane and 
1 ,3-dichloropropene is an effective soil 
amender. The discovery of its efficacy 
came about in an interesting w'ay. 

The mealybug wilt of pineapple had 
been seen to be much less serious in 
virgin lands in Hawaii; the point was 
confirmed in other tropical countries. 

As a result, a continuous search was 
made for soil amenders that might re- 
store some of the qualities of virgin soil 
that produced more wilt-resistant pine- 
apple plants. The study had gone on 
more than 5 years with no satisfactory 
results, when a number of chlorinated 
hydrocarbons were provided by the 
Shell Development Co. for trial. None 
of them had any effect on the suscepti- 
bility of pineapple plants to mealybug 
wilt, but one of them, the mixture I re- 
ferred to, which now is known as D-D 
mixture, proved to be the most prac- 
tical and successful soil amender known 
up to that time. 

The first results with pineapple plants 


127 

were available shortly after the out- 
break of the Second World War, when 
the domestic production of vegetables 
became of great importance. Soil treat- 
ed with D-D mixture and planted to 
carrots and other vegetables produced 
much more heavily than nontreated 
check plots. The result undoubtedly 
was due to the measure of control of 
nematodes that had been achieved. 

D-D thus proved to be a most effec- 
tive nematocide, although the discov- 
ery was purely by chance. Perhaps that 
was all to the good, for it gave an op- 
portunity for the soil -amendment qual- 
ities of the material to be recognized 
early in its development. A logical con- 
sequence was the added recognition of 
growth response beyond that due to 
nematode control as one basic require- 
ment for an effective soil fumigant. 

Hawaii has also pioneered in the 
development of suitable injection ma- 
chinery. Injection is a problem when 
large acreages have to be treated and 
planted in a short season. Probably the 
first large-scale field fumigation ma- 
chine was the one engineered by the 
California Packing Corp. for use with 
chloropicrin. The development of the 
field injectors was not easy. D-D is rela- 
tively corrosive and requires special 
metals. Pumps and delivery systems 
had to be devised — and then rede- 
signed to get the most efficiency. The use 
of check rows has iong been dropped 
as unnecessary in pineapple fields, 
but many an example is still provided 
unwittingly when application is faulty 
and long rows or partial rows are left 
untreated. From them the increasing 
necessity for soil fumigation, as time 
goes on, is demonstrated. 

The methods available for small 
growers of truck crops have been 
greatly improved by the development 
of more effective hand injectors by 
firms on the United States mainland. 
With those new methods and new ma- 
chinery, D-D and other fumigants, 
such as ethylene di bromide, have l>een 
found to be economical and practical 
as nematocides and as soil amenders. 



128 


YEARBOOK OF AGRICULTURE IfSB 


The use of D-D mixture has become 
standard practice in Hawaii on pine- 
apple lands. Some 7 million pounds 
are used in that way each year. The 
fact that in 1942, when the first results 
were obtained, only laboratory quan- 
tities were available as byproducts 
from a pilot plant used for other syn- 
theses underscores the remarkableness 
of the development. Furthermore, the 
total volume of fumigants used on a 
held scale is evidence that Hawaii has 
pioneered in a development of vast 
significance to agriculture. 

Perhaps a more important result of 
the discovery of D-D mixture was the 
stimulus given to the whole problem 
of soil amendment by fumigation for 
field crops in the United States and 
in many other countries. Other fumi- 
gants, particularly ethylene di bro- 
mide, have appeared on the market 
and are competitive with D-D mixture. 

Some ethylene dibromidc has been 
used in Hawaii on pineapple soils as 
a preplanting fumigant in place of 
D-D, An exact evaluation of the rela- 
tive merits of the two compounds for 
the purpose is difficult because EDB 
is more sensitive to soil-moisture con- 
ditions than is D-D. With appropriate 
soil moisture, EDB has given excellent 
response. As most of the pineapple 
acreage is planted during dry seasons, 
however, D-D is perhaps the most 
reliable general preplanting fumigant. 
EDB has found a place in the post- 
planting fumigation of pineapple fields. 
Ethylene chlorobromide (ECB) is also 
promising for this purpose. The process 
involves some risk to the growing plant 
but growth stimulation usually has 
been pronounced. Sometimes profit- 
able increases in fruit weight have 
followed. 

Methods of testing soil fumigants 
have been dominated by the micro- 
biologists’ need for data on specific 
organisms, and the small pot test has 
been standard. New fumigants usually 
are screened by that method. Quan- 
titative result have accrued, but the 
interpretation of the results in terms 
that the grower can use is difficult, 


for the method at best is artificial and 
of too short duratiem. Field-plot tests 
furnish a more reliable criterion for 
the growers because ultimate crop 
yield must determine the economic 
feasibility of the practice. 

Future advances will come by under- 
standing how fumigants afiect growth. 

There is, first, the effect on specific 
organism-nematodes, soil insects such 
as wireworms, and bacteria and fungi, 
both pathogenic and beneficial. 

Second, there is growth stimulation. 
Plants may be stimulated because the 
development of root systems is has- 
tened and improved, either by re- 
moving root pathogens or by supply- 
ing necessary factors for their growth. 
Possibly there is release of root-pro- 
moting hormones in the soil. 

Nutrients may be more readily avail- 
able because of depression of the nitri- 
fying organisms in the soil. That is 
true of the early stages of growth, 
but growth stimulation of pineapple 
plants continues sometimes for the 
whole 4-year growth period and is 
often more pronounced in the second 
crop than in the first. Furthermore, 
soil fumigation after the plant has been 
established for several months will 
favorably affect the root system by 
stimulating or permitting new active 
white root tips for that portion of the 
whole root system that is near the 
point of injection of the fumigant. 
This suggests the possibility that soil 
fumigation makes nutrients available 
that are needed in small quantity for 
vigorous plant growth. 

These problems of growth stimula- 
tion are closely related to a third 
consideration; namely, the effect of 
the fumigant on fertilizer practices. 
That is a practical point because the 
effects may govern dosages to be used 
and the economic position of the chem- 
ical in the production of the crop. 

Walter Carter is a graduate of 
Montana State College and holds advanced 
degrees from the University of Minnesota, 
He is head of the entomology department of 
the Pineapple Research Institute of Hawaii, 



MORE ABOUT THE CONTROL OF NEMATODES 


More About the 
Control of 
Nematodes 

Albert £. Taylor 

The control of nematodes requires 
clean soil, clean planting stock, and 
sanitation. Plant parasitic nematodes 
arc eliminated from the soil by crop 
rotation, chemicals, heat, bare fallow, 
and a few other methods. 

The use of crop rotations to control 
nematodes is based on the fact that 
nematodes are obligate parasites and 
can neither live indefinitely nor re- 
produce unless they can feed on 
living plants. Furthermore, all have 
a degree of host specialization: Any 
given species can feed and reproduce 
only on certain plant species. Without 
those plants, the nematodes starx^e or 
succumb to parasites, predators, and 
diseases, even though other plants are 
grown nearby. 

The main disadvantage of crop ro- 
tations for control is the time required 
and the loss of income if the rotation 
crops arc less profitable than the main 
crop. When crops of low or moderate 
value are concerned, it is the only 
practical method of control. 

Chemicals used to kill nematodes in 
the soil must be efficient for killing the 
nematodes and must leave no residue 
that can harm plants. They should be 
easy to apply and inexpensive. Many 
chemicals have been tested for the 
purpose. Four arc in general use: 
Chloropicrin and mixtures that con- 
tain methyl bromide, chlorobromo- 
propene, ethylene dibromide, or di- 
chloropropcnc. One type of methyl bro- 
mide mixture is a gas at ordinary tem- 


129 

peratures. The others are liquids. The 
liquids are applied by injection into 
the soil. The methyl bromide gas is 
released under a cover placed above 
the soil. In either case, the fumes 
permeate tlie upper layers of the soil, 
killing the nematodes by contact. 

Each has advantages and disadvan- 
tages. Prices also vary, so the choice of 
one for any given plot of soil involves 
consideration of the organisms to be 
controlled, the local conditions, and 
the relation of the cost of the fumigant 
to the value of the crop. 

Chloropicrin applied at 200 pounds 
an acre is an excellent ncmatocide 
and insecticide. Applications of two or 
three times that amount also control 
some fungi, bacteria, and weeds. Be- 
cause chloropicrin does not penetrate 
undecayed plant material readily, it 
should be used only after the residues 
of a crop have had time to decay. 
Because its fumes in the air will dam- 
age plants, chloropicrin cannot be used 
in one part of a greenhouse while 
crops are growing in nearby sections — 
a difficulty that may also be en- 
countered outside. 

After chloropicrin is applied, it must 
be confined to the soil. The usual 
method i.s to apply a “water seaP’ by 
sprinkling with enough water to wet 
the top inch or two of soil. An interval 
of 5 to 25 days must be allowed 
between application of the fumigant 
and planting. The exact time depends 
on the amount applied, the soil mois- 
ture, and the type of soil. 

Extreme precautions must be used in 
handling chloropicrin. Small amounts 
of the fumes in the air will cause 
profuse watering of the eyes. Larger 
concentrations cause violent coughing, 
vomiting, or even death! Nevertheless, 
chloropicrin is not dangerous to use. 
In fact, the opposite is true to a 
certain degree, because the watering 
of the eyes gives preliminary warning 
of the presence of fumes before the 
more .serious effects ensue. No one 
will voluntarily remain in even a low 
concentration of chloropicrin fumes. 
Gas masks of the proper type give full 



YEARBOOK OF AGRICULTURE 1R53 


130 

protection. Chloropicrin is not in- 
flammable. The shipping containers 
are heavy cylinders for large quantities 
and sealed cans for i -pound bottles. 

The 98 percent methyl bromide fu- 
migants, applied at rates of i or 2 
pounds for each 100 square feet, give 
good control of nematodes, soil insects, 
and most weed seeds, fungi, and bac- 
teria. The 10 to 15 percent methyl 
bromide mixtures are good ncmato- 
cides and insecticides at the rate of 78 
to 100 gallons to the acre. Methyl 
bromide penetrates undecayed roots 
readily. Small amounts of fumes in the 
air do not damage growing plants. 
Crops can be planted 2 to 4 days after 
application. It is particularly useful for 
greenhouse fumigation. If reasonable 
precautions arc taken, methyl bromide 
is neither unpleasant nor dangerous to 
use. The 98 percent methyl bromide 
fumigants are obtainable in i -pound 
cans. The lo-perccnt or 15-percent 
mixtures are shipped in drums. 

Ethylene dibromide soil fumigants 
usually contain 41 percent to 83 per- 
cent of the chemical by weight, the 
diluent being naphtha. I’he rates usu- 
ally recommended to control nema- 
todes and soil insects are 10 to 20 gal- 
lons an acre of the 41 -percent mixture 
and proportionately less of the 83-per- 
cent material. The latter is often dilu- 
ted for convenience in application. 
Penetration of undecayed crop residues 
is good and small concentrations of 
fumes in the air are not toxic to plants. 
No water seal or cover is necessary. 
The soil can be planted 10 to 14 days 
after application of the fumigant. Eth- 
ylene dibromide fumigants arc not 
dangerous or unpleasant to handle if 
used with care. The .shipping contain- 
ers are drums of various sizes. 

Dichloropropene fumigants have 
about 50 percent of this chemical mixed 
with dichloropropane. Applications of 
20 gallons an acre are used against 
nematodes and soil insects. The kill of 
nematodes in undecayed crop residues 
is good. Fumes in the air do not injure 
plants. At least 2 wrecks must be al- 
lowed between application of the fumi- 


gant and planting of the crop. No cover 
or water seal is necessary. The shipping 
containers are steel drums. 

Chlorobromopropcne can be used 
effectively against nematodes, insects, 
and soil fungi. 

All soil fumigants are poisonous to 
man and animals. They must be han- 
dled with care lest the liquid come in 
contact with the skin or clothing and 
the fumes inhaled. If the fumigants are 
accidentally splashed on the skin, they 
should be washed off immediately with 
soap and water. If clothing or shoes be- 
come w-et, they should be removed in- 
stantly and not worn again until clean. 
Stored fumigants should be kept tightly 
scaled. With the ones that are inflam- 
mable or have inflammable diluents, 
precautions should be taken against 
fire or explosion. Most of the soil fumi- 
gants are corrosive to metals, particu- 
larly in moist air, so applicators should 
alw^ays be cleaned thoroughly after 
use and partly empty containers should 
be tightly closed. 

Besides fumigants, a few other ma- 
terials have limited use in the control 
of nematodes, the greater part of the 
applications being to soil u.sed for to- 
bacco and other seedbeds. Urea con- 
trols nematodes when applied at rates 
of 8 ounces to i pound the square yard 
and is often mixed with calcium cyana- 
mide for weed control. Sodium azide is 
also an effective nernatocide when used 
at the rate of 4 ounces to the square 
yard. I’hcse materials are used in pow- 
der form, which is mixed with the up- 
per layers of the soil. 

Liquid .soil fumigants arc put 6 to 
8 inches beneath the soil surface. The 
applications are made 10 to 12 inches 
apart horizontally. On a small scale 
that can be done with improvised 
equipment, but large-scale applica- 
tions require special applicators. For 
areas of less than an acre, hand appli- 
cators that have a calibrated pump to 
deliver measured amounts of the fumi- 
gant through a hollow spike thrust 
into the soil arc satisfactory. For larger 
areas, applicators drawn by tractors 
or mounted on tractors arc used. 



MORi Atour THI CONTROL OF NEMATODES 


They arc of two general types. One 
type delivers the fumigant in contin- 
uous streams behind shanks that run 
through the soil. The other delivers 
the stream of fumigant ahead of a 
plow which immediately turns the 
soil to cover it. If a shar^ applicator 
is used, the soil is prepared in advance 
by plowing, harrowing, and leveling. 
If a plow applicator is used, the har- 
rowing and leveling follow immedi- 
ately after application. Shank applica- 
tors can be made in any convenient 
.size, but usually have six to eight 
shanks and can cover an acre or more 
an hour. 

The essential points in soil fumiga- 
tion arc good preparation of the soil, 
application of the exact amount of 
fumigant desired with correct spacing 
and at the proper depth, and prompt- 
ness in carrying through the opera- 
tions necessary after application. Soil 
preparation includes cutting up of 
weeds, trash, and crop residues, which 
might interfere with the smooth oper- 
ation of the applicator. After the 
application of the fumigant, the soil 
should be left smooth with all clods 
well broken up. This is usually accom- 
plished by a drag behind the shanks 
of the applicator or by a harrow and 
drag after use of a plow applicator. 

Gaseous fumigants, such as 98 
percent methyl bromide, arc applied 
in a different manner. The soil is 
prepared as for planting. A gas-im- 
pervious cover, usually a specially 
treated paper, such as Sisalkraft, or 
plastic tarpaulins (Fumi Cover and 
others), is placed over the area to l^c 
fumigated. The cover is not in con- 
tact with the soil surface, but is sup- 
ported a few inches above it. The 
edges of the cover arc buried. The 
methyl bromide is then introduced by 
means of a plastic tube into an open 
container placed underneath this cover 
on the soil surface. Inexpensive appli- 
cators especially made for the purpose 
make it a simple operation. The cover 
is left in place for 24 to 48 hours. This 
method is limited to rather small plots. 


131 

It is used mostly to fumigate seedbeds, 
greenhouses, and nursery plots. 

When the best possible control of 
nematodes or other soil pests is de- 
sired or when the crop is to be planted 
in rows less than 24 inches apart, the 
fumigant is applied to the whole area 
to be planted. This is called solid, 
area, over-all, or broadcast fumigation. 

Row fumigation is used where crops 
arc to be planted in rows more than 
24 inches apart. One or two lines of 
fumigant are centered on the row. It 
requires some definite method of mark- 
ing the rows so that they can be 
located for planting. The usual pro- 
cedures are to form a raised bed when 
the fumigant is applied, to mark the 
rows by shallow furrows, or jto locate 
them w'ith reference to the tracks left 
by a tractor applicator. 

Strip applications of fumigant may 
be used when orchards arc to be 
planted. A strip of soil 6 to 8 feet wide 
is fumigated for each row of trees. 

Site fumigation is used in orchards 
or when individual trees or shrubs arc 
to be planted. An area 6 to 8 feet in 
diameter centered on the planting 
spot is fumigated by means of a hand 
applicator. 

If crops are to \>e planted in widely 
spaced hills, spot fumigation can be 
used. The locations of the hills are 
marked and the fumigant placed with 
hand applicators. 

The advantage of strip, row, site, 
or spot fumigation is the saving in the 
amount of fumigant required. At 
the .same lime the plants are protected 
from serious infection wiien they arc 
small and most vulnerable. Often they 
are the most advantageous methods 
of using soil fumigants. 

Best results with soil fulnigation are 
had on soils of the lighter types, such 
as sandy loams. Fumigation of heavy 
soils is often disappointing. Muck or 
peat soils require an increase of 50 
percent to 100 percent in the amount 
of fumigant applied to obtain results 
comparable to those obtained on 
light mineral soils. The soil should be 
neither very wet nor very dry when 



YiARBOOK OF AORICULTURI 1953 


132 

the fumigant is applied, but should 
have a moisture content about right 
for planting seed. The soil temperature 
at a depth of 6 inches should be above 
50® F., though some fumigants can be 
applied at temjDeratures as low as 40®. 

As all of the fumigants are somewhat 
toxic to plants, they must be applied 
far enough in advance of planting to 
allow them to have their effect and 
then diffuse out of the soil. This aera- 
tion time depends on the type and 
amount of fumigant used, the type of 
soil, and the temperature and mois- 
ture conditions. Fumigants disappear 
more quickly from warm soils than 
from cold soils and more quickly from 
dry or moist soils than from wet soils. 
Plowing or otherwise working the soil 
will hasten aeration, but should not be 
done until at least a week after appli- 
cation. It should be emphasized that 
the number of days l)etween applica- 
tion of fumigant and planting as given 
in the preceding discussion are mini- 
mum times. It is often convenient to 
fumigate the soil .several months in 
advance of planting. Soil can be fumi- 
gated in the fall for planting in the 
spring without serious loss of efficiency. 

The effect of succc.s.sful .soil fumiga- 
tion is the elimination of enough of the 
nematodes and other soil pests so that 
the crop is not seriously damaged. 
Since the damage from nematodes is 
usually the formation of galls or the 
partial destruction of the root system, 
there will be a marked increase in the 
number and extent of the roots, im- 
proved growth and vigor of the plant, 
and a tendency toward more uniform 
growth. If other conditions are favor- 
able, yields will increa.se. Yield in- 
creases of .several hundred percent are 
not uncommon when heavily infested 
soil Is fumigated, but usually yield 
increases are from 20 percent to 50 
percent. With root or tuber crops, the 
percentage of culls drops. Such liene- 
ficial effects are most conspicuous on 
the first crop following fumigation, but 
are often seen on subsequent crops. 

, Optimum applications of soil fumi- 
gants are not assumed to eradicate the 


nematodes but to provide the degree of 
control that will produce the most cash 
return in proportion to the cost of 
the fumigation. Usually it is better 
to use a moderate amount of fumigant 
for each crop than to attempt a higher 
degree of control with a larger amount 
in the hope that several crops can be 
raised before it is necessary to repeat 
the fumigation. 

Ethylene dibromide and dichloro- 
propene fumigants are the least ex- 
pensive. The average cost of moderate 
application.*: is about 35 or 40 dollars 
an acre. The two fumigants arc widely 
used on crops of moderate to high 
value when the increase of salable 
produce will be more than twice or 
three times the cost of the fumigation. 
The cost of applying 500 pounds of 
chloropicrin to the acre is 400 dollars. 
Liquid methyl bromide fumigants ap- 
plied at the usual rates cost about 175 
dollars an acre, and 98 percent methyl 
bromide fumigants cost about ^ 
cents per 100 square feet when i 
pound per 100 .square feet is applied. 
The use of these fumigants is confined 
to crops of very high value, to green- 
houses, seedbeds, and nurseries. Weed 
control by fumigation, since it elimi- 
nates expensive hand weeding, is often 
an important consideration. 

The principal question in the prac- 
tical use of soil fumigants is that of the 
possible profit to be obtained from 
their use — whether the increase in 
salable crops as a result of the fumi- 
gation will pay for the cost of the 
fumigant, the expense of application, 
and a reasonable profit. The best guide 
is experience with similar crops and 
conditions. Lacking that, trial plots 
can be used to compare yields from 
fumigated or unfumigated areas and 
to compare different fumigants or 
different rates of application of one 
fumigant. Such trials are advised if 
plant parasitic nematodes are known 
to be present in significant numbers, 
if yields of crops have declined over a 
period of years, and if the growth of 
crops is not uniform or root systems 
are poor. 



MORE ABOUT THE CONTROL OF NEMATODES 


All plant parasitic nematodes are 
killed almost instantly when heated 
to about 140® F. Several methods of 
heating soil for nematode control have 
been devised. The most common is 
steam, released from pipes buried in 
the soil or under a steam pan. Steam 
pans, usually of metal, are about 8 
inches deep and 6 to 8 feet wide by 
8 to 10 feet long. They arc closed 
above and open below. The edges of 
the pan are buried 3 or 4 inches deep. 
Steam is released under the pan until 
the top 6 or 8 inches of the soil is 
heated to the required degree. 

Other methods of applying heat arc 
used with small lots of potting soil, 
which are heated by steam in a closed 
chamber, exposed to dry heat in 
shallow layers, drenched with hot 
water, or heated by electricity. 

Many species of plant parasitic 
nemat<^es can be killed by drying. 
Small lots of soil can be air-dried by 
spreading out in thin layers. In 
favorable climates, the method can be 
applied bn a large scale, the usual 
procedure being repeated plowing of 
the soil during the dry season of the 
year. In any climate it is good practice 
to remove the roots of a nematode- 
infected crop from the soil as soon 
after harvest as possible and to allow 
them to dry before plowing under. 

In low, flat fields, flooding is some- 
times used. We have little information 
on the effect of the method, but farm- 
ers who use it generally agree that 
flooding for several weeks is necessary. 

Another possible method is bare fal- 
low. Keeping the soil free of all 
vegetation deprives the nematodes of 
the opportunity to feed and reproduce. 
But because of the labor required for 
weed control and possible deleterious 
effects on the soil, this is seldom 
practical even for small plots. 

One of the main sources of nematode 
infestation is planting stock, par- 
ticularly plants used for transplanting, 
bulbs, tubers, corms, and roots. Some 
species of nematodes, such as the 
wheat nematode, Angmna tritici^ and 
related forms, may be locat^ in 


133 

galled kernels mixed with seed, cysts 
of the genus Heterodera may be mixed 
with seed, or nematodes such as the 
rice nematode {Aphelenchoides oryzae) 
may be found between the seed and 
its enclosing glumes. Soil clinging to 
roots of transplants may be infest^. 

Other important sources of nema- 
tode infestation are soil brought into 
a field on vehicles or farm implements, 
or washed in by running water. 
Compost made from infected plants 
may also be infested. If a field is 
fumigated, special care should be 
taken to see that the seedbed is also 
fumigated. In fact, seedbed fumigation 
or sterilization by other means is ex- 
cellent practice under any conditions. 

It is best to discard infected planting 
stock, although it is sometimes possible 
and worth while to attempt to kill the 
nematodes in it and so save exception- 
ally valuable material. The hot-water 
treatment is extensively used for 
killing nematodes in narcissus and 
other bulbs, which, especially when 
dormant, can stand enough heat to 
kill the contained or adhering nema- 
todes without serious harm to the 
plants themselves. Narcissus bulbs arc 
presoaked in water with a wetting 
agent added, placed in water heated 
to 1 10® F. for 4 hours, and immediately 
dried or planted. Similar treatments 
have been worked out for other bulbs 
and planting stock. 

Attempts to kill nematodes in plant- 
ing stock by means of chemicals have 
been made, but always — so far — with 
serious injury to the plants. When the 
nematode is one of the ectoparasitic 
species — that is, one that does not 
enter the plant — it can be removed 
from transplants by simply washing 
off the adhering soil with cold water. 

Only a few satisfactory and practical 
methods of controlling nematodes on 
growing plants arc known. Such 
methods would find widespread use 
in orchards, in growing perennial 
shrubs, and even for annual crops. 
Where orchards or perennial orna- 
mentals arc to be planted, the only 
precautions that can be taken are to 



YIARBOOK OF AORICULTUtI 1f53 


134 

make sure that the soil is not infested 
before planting and that the trans- 
plants are free of nematodes. 

The type of cover crop used in 
peach orchards can have a consider- 
able effect on the degree of attack by 
root knot nematodes and consequently 
on the growth and yield of the trees. 
In experimental plots in Georgia, 
trees on plots where root knot resistant 
cover crops were planted produced 
about six times as many peaches in 
four seasons as trees on control plots 
where cover crops highly susceptible 
to root knot were planted. Where no 
cover crops at all were used and the 
plots were kept free of weeds, about 
five times as many peaches were 
produced as on the control plots. 

Some species of the nematode Aphe- 
lenchoides^ which parasitize the above- 
ground parts of such plants as straw- 
berries and chrysanthemums, can be 
controlled by repeated spraying of the 
plants with parathion. 

Undoubtedly the simplest method 
of preventing nematode damage is the 
use of plant varieties or rootstocks 
which are not susceptible to attack. 
Examples are the Shalil, Yunnan, 
and S-37 peach rootstocks, which are 
highly resistant to attack by some of 
the most common species of root 
knot nematodes in this country, 
though not to all of the root knot 
nematode species. Some advances 
have been made in the development 
of varieties of other crops resistant to 
root knot and other nematodes, but 
progress is necessarily slow and it will 
be many years before satisfactory 
nematode-resistant varieties of all 
crops will be available. 

Albert L. Taylor joined the division 
of nemaiology investigations of the Bureau 
of Plant Industry^ Soils, and Agricultural 
Engineering in 1^35. He did experimental 
work on soil fumigation in Tifton, Ga,, 
until 13^ when he joined the Shell Chemical 
Company to do research and development 
work on the soil fumigant D-D, He returned 
t9 the division of nemaiology in and is 
now stationed in Beltsville, Md, 


Treating Seeds 
To Prevent 
Diseases 


R. IV. Leukel 

Sometimes chemicals are applied to 
seeds, bulbs, corms, tubers, and roots 
to prevent their decay after planting 
and to control seed-borne and soil- 
bome plant diseases. 

To be satisfactory, a seed treatment 
has to be effective yet reasonably safe 
from seed injury in case of overdosage; 
economical, readily available, easily 
applied, and chemically stable; and 
not overly poisonous or disagreeable 
to operators or corrosive to metal. 

Fungicides may be classified as seed 
disinfestants, disinfectants, or pro- 
tec t^ts, according to the location of 
the organisms to be combatted. 

Disinfestants inactivate organisms, 
such as bunt spores, that arc borne on 
the surface of the seed. 

Disinfectants are effective against 
those located deeper within the seed. 

Protectants protect the seeds from 
attack by organisms that are present 
in the soil. 

Practically all effective seed-treat- 
ment materials are disinfestants. Many 
are also disinfectants and protectants. 
The formaldehyde and hot-water treat- 
ments, however, are disinfestants and 
disinfectants but are not seed pro- 
tectants. In fact, seeds that have been 
treated with formaldehyde or hot 
water frequently are attacked by 
soil-borne fungi more severely than 
are untreated seeds and therefore 
should be treated also with a pro- 
tectant before planting. 



TIEATINO SEEDS TO PIEVENT DISEASES 


Based on composition, fungicides 
may be organic or inorganic, mer- 
curial or nonmercurial, and metallic 
or nonmetallic. There arc organic 
mercurials (Cercsan) and inorganic 
mercurials (calomel); there are non- 
mercurial metallic organics (Fcrmatc) 
and nonmetallic organics (Spergon); 
there arc metallic inorganics (copper 
carbonate) and nonmetallic inorganics 
(sulfur). 

Fungicidal seed treatments may be 
dry or wet according to the form in 
which the fungicide is applied to the 
seed. 

In a dry treatment, the fungicide is 
applied in dust form, usually in a 
mechanical mixer at rates ranging 
from to 4 ounces or more to the 
bushel. 

Wet treatments once meant soak- 
ing the seed in a water solution of the 
fungicide for a certain period, after 
which the seed was allowed to drain 
and dry. Wet treatments now are ap- 
plied mostly by the slurry method or 
the “quick-wet” method. 

In the slurry method, the fungicide 
is applied to the seed as a soup-like 
water suspension, which is mixed with 
the seed in a special slurry treater. 
The seed requires no drying but may 
be bagged immediately for sowing or 
storage. 

In the “quick-wet” method, a con- 
centrated solution of a volatile fungi- 
cide is applied to the seed and 
thoroughly mixed with it. The dosage 
may range from K to 5 fluid ounces 
to a bushel. As in the slurry treatment, 
that adds less than i percent of mois- 
ture to the seed. The well-known 
formaldehyde spray treatment of oats 
is essentially a “quick-wet” treatment. 
So also is the method recommended 
for applying Panogen, Mercuran, 
Setrete, and several other materials. 

Inorganic mercurials used for treat- 
ing seed are limited practically to 
mercuric chloride, mercurous chloride 
(calomel), and mercuric oxide. 

Mercuric chloride, as a i to 1,000 
solution, may be used for treating po- 
tato se<^ pieces, sweetpotatoes, and 


«35 

rhubarb roots for planting. It is also 
used for seed of crucifers (plants of the 
mustard or cabbage family), celery, 
cucumber, pepper, tomato, water- 
melon, and certain other vegetables. 
Most seed.s arc more or less susceptible 
to injury by mercuric chloride. 

Calomel is used on seeds of crucifers, 
celery, and onion. Mercuric oxide may 
be used as a dip treatment for sweet- 
potatocs (i pound to 30 gallons of 
water). 

Organic mercurials arc more nu- 
merous and more widely used than the 
inorganics just mentioned. They are 
used on seed of small grains, legumes, 
grasses, cotton, beets, flax, sorghum, 
and some other field crops, and also on 
certain corms, bulbs, tubers, and roots 
and the seeds of some vegetables. 

Oresan, 2 percent ethyl mercury 
chloride, introduced in 1926, was the 
first organic mercurial widely used in 
the United States. It is applied at 2 
ounces a bushel. It was followed and 
largely replaced in 1933 by New Im- 
proved Ctresan, 5 percent ethyl mer- 
cury phosphate, which is applied at J4 
ounce per bushel. Both were used 
mostly on small grains, flax, cotton, 
peas, hemp, and sugar beets. 

Cercsan M, 7.7 percent ethyl mer- 
cury /?- toluene sulfonanilidc, appeared 
in 1948. It largely replaced the two 
previous Ccresans because of several ad- 
vantages over them, including its ap- 
plication as a slurry. 

Leytosan and Agrox, 7.2 percent and 
6.8 percent phenyl mercury urea, re- 
.spectively, are applied to small grains, 
peas, rice, and sorghum at ounce to 
the ljushel and to flax at ounces. 
They may be applied in dust or slurry 
form. 

Mercuran, 3.5 percent mercury as 
methoxy ethyl mercuric acetate, is 
used at the rate of ounce per bushel 
on small grains. It may be applied as 
a dust, in concentrated solution by the 
“quick-wet” method, or in a more di- 
lute solution with a slurry machine. 

Panogen, 2.2 percent methyl mer- 
cury dicyan diamide, is a concentrated 



YiAllOOK or AORICUITURI lfS3 


136 

liquid applied at % fluid ounce per 
bushel to small grains, 1% fluid ounces 
to flax, and 4 fluid ounces per 100 
pounds of segregated beet seed. It is 
applied in a special Panogen treater, 
but can be successfully applied in a 
slurry treater if diluted with water. 

Setrete, 7 percent phenyl mercury 
ammonium acetate, is a concentrated 
liquid that may be applied as such at 
}i ounfce per bushel, or it may be 
diluted I to g with water and applied 
in a slurry treater. 

Mersolite, 5 percent phenyl mercury 
acetate, is u^ as a dip treatment (i 
pound to 800 gallons) for narcissus 
corms to combat basal rot. 

Merthiolate, sodium ethyl mercury 
thiosalicylate, is used to prevent corm 
rot and yellows in gladiolus. 

Sanoseed, 7.9 percent ethanol mer- 
cury chloride, and Corona P. D. 7, 5 
percent mercury in a mercury bro- 
mine-phenol compound, arc used as 
dip treatments for seed potatoes. 

Semesan, 30 percent hydroxy mer- 
curic chlorophenol (19 percent Hg), is 
an excellent mercurial used as a wet 
soak treatment for bulbs, tubers, and 
corms and as a dust treatment for seeds 
of flowers and vegetables. 

Semesan Bel, a mixture of 2 percent 
hydroxy mcrcurichlorophenol and 12 
percent hydroxymercurinitrophenol, is 
used as a dip treatment for seed 
{x>tatoes. 

Puratized N-5-E, 10 percent phenyl 
mercury triethanol ammonium lactate, 
is used for treating seed potatoes and 
lily bulbs. 

L~224, an experimental mcrcury- 
zinc-chromate material, is an excellent 
treatment for seed com. 

Aagrano, 3.5 percent ethoxy propyl 
mercury bromide, is cflcctive against 
cereal diseases, especially when it is 
applied in slurry form. 

Semenon, 2 percent isopropyl methyl 
mercury acetate, gave excellent results 
in controlling diseases of small grains 
and sorghum. Both Aagrano and Se- 
menon arc European products. They 
were not available in the United States 
in I953« 


Nonmercurial organic fungicides 
have increased greatly in number since 
1945. Generally they are less eflective 
than the mercurials, but as a rule they 
are less injurious to seeds and less dan- 
gerous to persons using them. The or- 
ganic sulfurs and quinones are promi- 
nent ingredients in these compounds 
and often arc combined with phenol, 
chlorine, bromine, quinoline, zinc, 
iron, copper, sodium, or other ma- 
terials. 

Spergon, 98 percent chloranil (tetra- 
chloro-/^benzoquinone), was among 
the first nonmetallic organics to be 
widely used for treating seed, espe- 
cially peas and beans. It is used for vege- 
table seeds, com, sorghum, peanuts, 
alfalfa, clover, soybeans, and some 
other crops. It may be applied as a 
dust or as a slurry. 

Arasan, 50 percent thiram (tetra- 
methylthiuram disulfide), still another 
early organic fungicide, is used for the 
same crops as Spergon. Both will also 
control bunt in wheat, but are not rec- 
ommended for treating oats or barley. 

Arasan SFX, 75 percent thiram, is 
the wettable form of Arasan for treat- 
ing seeds by the slurry method. Tersan, 
also a wettable form of thiram, is used 
for the control of diseases of turf and 
lawn grass. 

Phygon (formerly Phygon XL) con- 
sists of 50 percent 2,3-dichloro-i,4- 
napthoquinone and 50 percent talc. 
It is an effective seed treatment 
for corn, peanuts, rice, sorghum, and 
most vegetables. It controls bunt in 
wheat, but is not recommended for 
other small grains. 

Zerlate, 70 percent ziram (zinc, di- 
methyl dithiocarbamate), is effective 
as a prebedding dip for controlling 
black rot in sweetpotatoes. It is similar 
to Zincate, Methasan, Zimate, and 
Karbam, as all contain ziram as the 
active ingredient. 

Fermatc, 70 percent ferbam (ferric 
dimethyl dithiocarbamate), like 21er- 
late, is used as a prebedding dip for 
sweetpotatoes. Both materials are tised 
alsQ as foliage dusts or sprays. 

Dow g-B, 50 percent zinc trichloro- 



TIEATINO SEEDS TO EEEVENT DISEASES 


phenate, has been used to treat gladi* 
olus bulbs and seed of cotton, corn, and 
sorghum. 

Dithane Z-78, 65 percent zinc ethyl- 
ene bisdithiocarbamate, has shown 
promise as a disinfestant and chemo- 
therapeutic fungicide. 

Mycon, 7.7 percent methyl arseni- 
lulfide, in extensive field tests, has been 
found effective in controlling those 
seed-borne diseases of wheat, oats, and 
barley that are amenable to control by 
fungicides. 

Seedox, 50 percent a,4,5-trichloro- 
phenyl acetate, has been used to treat 
cottonseed. Mycotox is similar to Seed- 
ox. Neither is effective as a seed treat- 
ment for small grains. 

Anticaric, 40 percent hexachloro- 
benzene, is effective as a seed treat- 
ment for the control of bunt in wheat. 
When applied to the soil it also pre- 
vents infection due to bunt spores in 
the soil. It is not recommended for 
treating seeds of other cereals. 

Pcntachloronitrobenzcnc (50 per- 
cent) controlled covered kernel smut 
in kafir and a 20-percent product con- 
trolled bunt in wheat. In Europe this 
chemical is reported as having con- 
trolled infection from soil-borne spores 
of both common bunt and dwarf bunt 
when it was applied to the soil at 
planting time at the rate of about 50 
pounds an acre. 

Inorganic nonmercurials are few. 
Copper carbonate, the first dust seed 
treatment to be widely used in agri- 
culture, and basic copper sulfate are 
still used on wheat as bunt preventives. 
Copper sulfate (bluestone) solution, 
once a popular seed treatment for 
wheat, now is used for that purpose to 
a very limited extent. 

Cuprous oxide (yellow or red) serves 
as a seed protectant for vegetable seeds 
to prevent seed decay and prccmcr- 
gcnce damping-off. It is injurious to 
seeds of lettuce, crucifers, and onions. 

Va^o 4, a mixture of zinc oxide and 
zinc hydroxide, is used on seed of 
crucifers, spinach, and other vegetables 
that are sensitive to cuprous oxide. 


137 

Other seed-treatment materials, 
some effective and some experimental, 
may be mentioned. The hot-water 
treatment remains the standard meth- 
od for controlling the flower-infecting 
loose smuts of wheat and barley. It is 
effective also for treating seed of cru- 
cifers, onion, tomato, and some other 
vegetables. 

Some gases, such as chlorine, have 
been suggested for treating large quan- 
tities of seed, but their effectiveness 
has not been proved. 

Hot vapor was described in 1944 as 
being applied to tons of seed exposed 
on a moving belt. Ultraviolet and in- 
frared rays, short waves, Hertzian 
waves, diathermy, X-rays, and other 
similar devices have been tried as seed 
disinfectants, but none has been 
proved practicable. Like hot water, 
these materials would not act as seed 
protectants, and so a supplementary 
treatment would be necessary to guard 
against soil-borne fungi. 

Testing the effectiveness of fun- 
gicides in the control of seed-borne 
diseases presents two chief difficulties: 
Obtaining a supply of suitable seed 
that is sufficiently infected to furnish 
an adequate test for the fungicides, 
and obtaining environmental condi- 
tions after planting that favor infection 
in the plants. 

In diseases like bunt of wheat, in 
which the causal spores are located on 
the surface of the seed, clean seed can 
be infested artificially if a supply of 
spores is available. But many disease 
organisms arc located deeper within 
the seed in a manner that cannot be 
duplicated artificially. So one has to 
get seed from a badly 'infected crop, 
or, better still, from the seed lot that 
produced that crop. At times seed ob- 
tained from a heavily infected field 
may be infected too lightly to serve as 
an adequate test for seed treatment 
because conditions for infection may 
have been very unfavorable at the 
critical period. 

We must observe certain precau- 
tions in testing fungicides for seed 



YEAIBOOK OF AORICULTURI I95R 


138 

treatment. The seed should be thor- 
oughly cleaned, before treatment, to 
remove dust, chaff, weed seeds, and 
other substances, all of which take up 
much of the fungicide. Proper dosage 
is important because the seed sample 
used usually is relatively small and 
hence the amount of fungicide applied 
must be carefully weighed or meas- 
ured. In treating cereal seeds exper- 
imentally, 500 cubic centimeters is a 
convenient sample. This volume, 
which is 1/70 of a bushel, simplifies 
the conversion of ounces-per-bushel to 
grams-per-sample. If the desired rate 
of application is one ounce (28.34 
grams) per bushel, 1/70 of a bushel 
will require 1/70 of 28.34 grams, or 
0.4 grams. Rates of }i, 2, 3, and 4 
ounces per bushel are easily converted 
to 0.2, 0.8, 1.2 and 1.6 grams per 
sample, respectively. Differences in 
bushel weight among different seeds 
or seed lots then can be ignored. It 
also avoids the error involved in 
treating samples of light, chaffy seed 
as compared with plump, heavy seed. 
The light seed should receive more 
fungicide for each weighed bushel than 
the heavy seed. 

When small samples of seed are 
treated, the capacity of the container 
should be such that it is only half filled 
by the sample. It should be first “con- 
ditioned” by treating in it a sample of 
seed at a rate sufficient to coat the 
inside with the fungicide. This seed is 
then discarded. 

After the fungicides have been ap- 
plied to the different samples, the con- 
tainers should be shaken in some 
mechanical contrivance so that all 
receive the same amount of mixing. 
Thorough mixing is especially es- 
sential when applying the nonvolatile 
materials. 

Between the treatment and sowing, 
the treated and untreated samples 
should be stored at a moderate tem- 
perature and preferably at a low 
humidity. Metal or glass containers 
are preferable to paper envelopes 
because if the envelopes are stored in 
contact with one another, the fumes 


from a volatile mercurial fungicide 
in one envelope will treat the seed 
in the adjoining envelope. If this 
envelope contains the untreated check 
sample, it will be rendered useless 
for that purpose. 

The effects of the treatments on 
germination of the seed, seedling 
emergence, disease control, and plant 
growth and yield are among the details 
usually desired from seed-treatment 
experiments. 

Germination tests, to study any 
harmful effects of the fungicides on 
the seed itself, may be made on wet 
blotters placed in incubators (in 
which temj^erature and humidity are 
controlled) or in steamed wet sand 
or soil. Relatively disease-free seed and 
disinfected soil should be used in the 
tests, because the harmful effects of a 
fungicide on the seed may be masked 
by its protective effect against sced- 
bome or soil-borne fungi that cause 
seed rot or preemergence damping-off. 

The use of infested soil is essential 
for determining the effect of the seed- 
protectant qualities of treatments on 
emergence. That may involve the iso- 
lation and culture of soil-bome fungi, 
such as species of Pythium^ Fusarium^ 
Helminthosporium^ and R/iizoctonia, and 
using the pure cultures to inoculate 
soil in which the treated (and un- 
treated) seed is to be planted. Such 
tests may be made in the greenhouse 
in pots, flats, or beds. The soil should 
be steamed or chemically fumigated 
before being inoculated in order to 
determine the effectiveness of the 
fungicides against each specific soil- 
borne fungus culture. 

The effectiveness of fungicides in 
the control of seed-borne diseases, 
such as the smuts of cereals, that are 
not apparent in the seedling stage 
usually is studied in field plots. The 
seed should be sown at the proper date 
so that the soil temperature as well 
as moisture before emergence are 
conducive to infection. For the cereals, 
that calls for soil that is not too moist 
for aeration and germination of the 
seed-bome spores. Along with this 



TREATING SEEDS TO PREVENT DISEASES 


somewhat submedium moisture con* 
tent of the soil, the temperatures con- 
sidered conducive to infection by 
cereal diseases are: Bunt of wheat, 
41® to 50® F.; barley covered smut, 
50® to 68®^ false loose smut of barley, 
59® to 68®; the smuts of oats, about 
64® to 72®; the barley stripe disease, 
46® to 59®; and kernel smuts of sor- 
ghum, 75® to 86®. Periods favorable to 
infection cannot be predicted with 
certainty. Frequently, because of the 
absence of such conditions, significant 
field data are not obtained. 

The effect of the seed treatment on 
yield should be obtained by treating 
and sowing relatively disease-free seed 
in replicated plots along with un- 
treated seed. Increases in yield from 
such treated seed presumably reflect 
the seed-protectant qualities of the 
fungicide used. 

Cereal seed treatment was rather 
widely practiced for quite a few years 
before the treatment of other crop 
seeds was generally recommended. 

The reasons perhaps were that 
smuts could be seen in cereals and that 
it was discovered early that some of 
them could be prevented by seed 
treatment. 

The beneficial effects of the treat- 
ment of cereal seeds may result from 
the elimination of seed-borne diseases, 
the prevention of seed rot and seedling 
blight, and the suppression of weeds 
by better and more even stands. 

One of the greatest benefits lies in 
the elimination of some seed-borne 
fungi or bacteria that cause primary 
infection lesions from which the dis- 
ease spreads to other plants. Out- 
standing examples are certain hel- 
minthosporium diseases of wheat, oats, 
and barley. This spread by secondary 
infection may cause heavy loss, al- 
though only a small percentage of 
the seed sown may have been in- 
fected. Annual seed treatment of 
cereal seed is now considered a wise 
farm practice because the use of 
disease-free or treated seed one year 
does not insure the production of 


139 

disease-free seed for the next year’s 
crop. Airborne spores from neigh- 
boring fields may contaminate the 
heads of grain grown from disease- 
free seed, so that seed from these 
heads, if sown untreated, may produce 
a diseased crop the following year. 
Growers of certified seed have found 
it wise to guard against this source of 
infection, as it may disqualify their 
fields for certification. 

Wheat is treated mostly for the con- 
trol of bunt, which if only seed-borne 
is the most easily controlled of all the 
seed-borne cereal smuts. Ceresan M, 
Agrox, Setrete, Panogen, Leytosan, 
Mercuran, Aagrano, and some other 
organic mercurial compounds are 
generally most effective in bunt con- 
trol, especially if infection is severe. 
Most are applied at less than an 
ounce to the bushel. Many nonmer- 
curials also are effective — copper car- 
bonate, basic copper sulfate, Arasan, 
Spergon, Phygon, Anticaric, Mycon, 
and several experimental materials. 
The mercurials by and large are pref- 
erable because they eliminate also 
some of the pathogens borne more 
deeply within the seed. Loose smut 
(caus^ by Vstilago tritici) is prevented 
only by the hot- water treatment. 

Rye may be treated to prevent the 
spread of seed-borne diseases, like 
stalk smut and bunt. The treatments 
for wheat may be used also for rye. 

Barley is treated largely for the 
prevention of covered smut, black or 
false loose smut, and stripe disease. 
Seed treatment also reduces the 
amount of primary infection from such 
diseases as bacterial blight, scab, net 
blotch, and spot blotch. The fiingi- 
cides recommended are restricted 
largely to the organic mercurials such 
as Ceresan M, Panogen, Leytosan, and 
Agrox. The nonmcrcurial organics 
may improve stands and reduce in- 
fection by these diseases to some ex- 
tent but, with a few exceptions, they 
seldom control them satisfactorily. 
The flower-infecting loose smut (caus^ 
by UstUago nuda) is controlled only by 
the hot-water treatment. 



140 YEARBOOK OF A 

Seed of oats, like that of barley, is 
treated most frequently for the pre- 
vention of the smut diseases, which are 
visible at heading time. The effective 
treatments are restricted largely to the 
organic mercurials, although the for- 
maldehyde spray treatment is widely 
used. It is cheap and effective but may 
injure the seed. Also it is not a seed 
protectant. The effective oi^anic mer- 
curials also prevent primary infectitm 
from seed-borne halo blight, frisarium 
blight^ anthracnose, and the helmin- 
thosporium diseases; they will not pre- 
vent these diseases, however, if the 
causal organisms are present in the 
soil. 

Corn, a warm-season crop, is subject 
to many diseases, most of which cannot 
be prevented by seed treatment. Dis- 
ease control or prevention is largely a 
matter of developing disease-resistant 
strains and providing favorable grow- 
ing conditions for the plants. The chief 
purpose of corn seed treatment is to 
prevent seed rot and seedling blight 
caused by seed -borne and soU-bome 
fungi, especially when cold, wet weath- 
er follows planting. For many years 
Semesan Jr. (i percent ethyl mercury 
phosphate) was most widely used for 
the purpose, but it has been supplanted 
largely by the nonmercurial organics, 
such as Arasan, Spergon, Phygon, and 
Dow 9“B. The experimental com- 
pound, L- 224, a mercury zinc chro- 
mate, and Dithane also have proved to 
be effective. Mercury compounds may 
injure com seeds that have been dam- 
aged near the embryo by rough han- 
dling, especially if planting is delayed 
after treating. 

Hybrid corn seed, which constitutes 
about 80 percent of the corn seed 
planted, is treated at the seed houses 
before it is sold to the growers; thus 
the work of seed treatment of corn 
mostly has been taken out of the hands 
of individual growers. 

Treatment of rice seed was not a 
generally recommended farm practice 
until about ^947. Experiments proved 
that the treatments increased stands, 
especially in the early seedings when 


ORICULTURE 1953 

the soil was cold and wet. Often 
yields also were improved. Best results 
were obtained with Ceresan M, Phy- 
gon, Arasan, and Spergon. Ceresan M 
prevented seedling blight caused by 
Helminthosporium oryzae^ Dow g-B in- 
jured the seed after long storage. 
Cuprocide (cuprous oxide) seemed 
best for seed sown in water, but it may 
injure presprouted seed. Rice may be 
fumigated with methyl t»x>mide to 
combat the seed-borne rice nematode. 
Exposure to a concentration of tK 
pounds of methyl bromide to r,ooo 
cubic feet of space for 12 to 15 hours 
will kill the seed-borne nematodes 
without injuring the seed seriously. 

Sorghum, like corn, is benefited 
most by seed treatment when cold, wet 
weather follows planting. It reduces 
seed rot and seedling blight and pre- 
vents infection by the kernel smuts. 
The nonrnercurial organics, such as 
Phygon and Arasan, have b^n found 
beneficial in improving stands and 
controlling smut In varieties whose 
seeds have persistent glumes, however, 
the kernel smuts are controlled more 
effectively by the use of volatile or- 
ganic mercurials, such as Ceresan M 
and Panogen. 

Sugar beet seed is treated mostly 
to combat sccd-borne infection by 
Pkoma betae and Cercospora beticola. 
Seed treatment is effective in pre- 
venting precmcrgcncc damping-off, 
caused either by seed- or soil-borne 
fungi. Materials used for beet seed 
treatment include the organic fungi- 
cides N. I. Ceresan, Ceresan M, 
Panogen, Phygon, and Arasan. In- 
organic mercury compounds, cuprous 
oxide, and various mixtures of mer- 
curials with copper carbonate have 
proved effective experimentally but 
never have come into widespread com- 
mercial use. 

Prccmergencc and postemergence 
damping-off have been successfully 
combatted in greenhouse experiments 
by applying an Arasan-fertilizer mix- 
ture to the soil so that the sugar beet 
seed germinated in the soil impreg- 
nated with the mixture and the seed- 



TRiATlHO SiRDS TO PtEVENT DISEASES 


141 


lings grew through it. The Arasan was 
used at a rate of about 4 pounds an 
acre. Field experiments to control 
black root by Arasan-fertilizcr mix- 
tures have not given consistently 
favorable results. 

Seed treatments for the control oC 
damping-olF caused by PyMum spe- 
cies, Phma biU^r and the Rhizt>eima 
species unless s^ in« 

is con* 

ditiosis ait u#tycrabk. Black root^^ 
caused by cochlmdes, how- 

ever, is not prevented by seed treat- 
ment. 

Proper soil management helps re- 
duce the soil populations of sugar beet 
pathogens. Adequate drainage and 
heavy application of commercial ferti- 
lizers, especially phosphate, are im- 
portant. Of great importance also is 
a rotation in which sugar beets do 
not immediately follow a legume sod 
crop but follow an early fall-plowed 
legume. Such handling of the legume 
crop is necessary because clovers, 
swectclovcr, and alfalfa harbor the 
various pathogens that cause damp- 
ing-off. Their sods, if spring-plowed, 
produce peak populations of the fun- 
gus at the period corresponding to 
planting time for sugar beets. If associ- 
ated with proper soil management, 
control of excessive soil moisture by 
drainage, and a good fertilizer prac- 
tice, seed treatments show value. 

Cotton, flax, and hemp respond to 
seed treatment in the order named. 
The diseases of cotton that arc re- 
duced somewhat by seed treatment 
arc bacterial blight or angular leaf 
spot {Xanthommas malvacearum)^ an- 
thracnose (Colleioirichum gossypii)^ sore 
shin (Rhizoctonia solani), and seedling 
blight caused by species of Aspergillus^ 
Fusariuniy Diplodidy Sclerotiumy and other 
fungi. 

^d of cotton is generally delinted 
before being treated because seed- 
borne infection is more easily elimi- 
nated in delinted seed. Delinting may 
be done mechanically by reginning or 
chemically by acid treatment. Me- 
chanical delinting may injure the seed 


and impair its germination. Acid- 
delintcd seed germinates better than 
fuzzy seed, but it rots more casUy, 
especially in cold, wet soil. Effective 
seed treatments largieVy prevent ihat. 
Organic mercurials 1 ,suc\l aa Vhie 
C«te«ua>> NOXVv tfSSS^ 
gateralSy been more effective tbm 

nonmcrcurial organics (such as Sper- 
|<w^ Dow o-B, Ky^on, Arasan, and 
beedox) in eliminating seed-bome 
infection. Some growers object to the 
use of poisonous fungicides, such as 
the mercurials, however, bt^use ex- 
cess treated seed may b^ome mixed 
with untreated seed used for making 
cottonseed meal or oil. The non- 
mercurials are especially useful as se^ 
protectants for acid-delinted seed. 

Treatment of flaxseed is made neces- 
sary largely because seed, especially 
of large-seeded kinds, may be injured 
in threshing. Many of the fractured 
seeds rot after planting, particularly 
in heavy soils, unless they arc first 
treated with an efficient protectant, 
which prevents invasion by species of 
Alternaridy Penicilliumy Fusariumy and 
Pythium, Several seed-borne diseases 
of flax ^re alleviated by seed treat- 
ment. Pasmo {Mycosphaerella linorum)^ 
when seed-borne, causes primary in- 
fection lesions, which initiate second- 
ary infection in other plants. Brown- 
ing and stem break {Polyspora lint) 
and anthracnose {Cdletotrichum lini- 
colum) also may be seed-borne. 

One of the difficulties in treating 
flaxseed is the failure of dry fungicides 
to adhere to the smooth seed coat. 
The seed therefore requires a much 
heavier dosage of dusts than is applied 
to most other seeds. Wet treatments 
cause gumming of the seed because of 
the mucillaginous coat. The organic 
mercury dusts usually are appli^ at 
i)4 ounces per btuhel. Nonmercurials 
are applied somewhat more heavily. 

Seed treatment of flax may increase 
stands, but increased yields do not 
always follow unless an abundance 
of weeds prevents sufficient branching 
to compensate for the thinner stands 
from untreated seed. 



YEARBOOK OF A O R I C U t TU R E 1 9 S 3 


142 

Treatment of hcmpsced with New 
Improved Ceresan, Spergon, and Ara- 
san was found to improve stands 
when planting was followed by un- 
favorable conditions for germination 
and growth. 

Treatment of seeds of forage crops 
controls some diseases, such as certain 
smuts in slender wheatgrass, millet, 
Canada wild rye, and Sudangrass. 
Seed treatment sometimes has in- 
creased stands in some species of 
LespedezCy Lotus ^ MedicagOy Meli lotus, 
and Ttifolium. Other species are in- 
jured by certain treatments when the 
treated seed is sown in dry soil. 

Seed of winter peas, mung beans, 
cowpeas, hop clover, hairy vetch, and 
alfalfa gave better germination, im- 
proved stands, and superior plants 
when treated with Spergon, Arasan, 
Phygon, or Dow 9-B in extensive field 
and greenhouse tests in 1949. Nodula- 
tion was not inhibited by treatment 
when the nitrogen-fixation culture was 
applied to the treated seed immedi- 
ately before sowing. Some investiga- 
tors, however, say that legume seed 
should not be treated before being in- 
oculated if it is to be sown in soil not 
previously cropped to legumes. 

Experiments with treating soybean 
seeds have been more numerous and 
extensive than with those of almost 
any other legume. In general, im- 
proved stands were had after the use of 
Arasan, Spergon, Phygon, and Dow 
9-B. Organic mercurials are some- 
times injurious. Increased stands due 
to treatment were not always followed 
by increased yields, probably because 
branching of the plants often compen- 
sates for thinner stands and because a 
higher percentage of the soybean 
flowers in a thin stand of plants will 
form pods. 

Treatment of peanut seed is a prof- 
itable farm practice, especially when 
mechanically shelled se^ is used. In- 
creases in stand have ranged from 30 
to 100 percent. Uninjured hand- 
shelled sect:? frequently gets no benefit 
from seed treatment except when un- 
favorable growing conditions follow 


planting. Arasan, Spergon, Phygon, 
Dow g-B, and Geresan M arc com- 
monly recommended. 

Vegetable seeds arc treated prima- 
rily to prevent seed rot and damping- 
off. Sometimes the control of seed- 
borne diseases is a major aim. The 
materials so used include Arasan, Phy- 
gon, Spergon, Fcrmate, Semesan, Dow 
9-B, N. I. Geresan, Geresan M, Gupro- 
cide, zinc oxide, zinc hydroxide, mer- 
curic and mercurous chlorides, copper 
sulfate, phenothiazine, Zerlate, Di- 
thane, and others. 

Arasan and Spergon are two widely 
used fungicides for vegetable seeds. 
Arasan seems most suitable for seed of 
beets, chard, and spinach. Spergon 
seems best for legumes. 

Some fungicides display differential 
benefit or injury toward the seed of 
certain crops. Cuprocide, for example, 
is injurious to seed of crucifers and 
lima beans and causes necrosis, de- 
layed absorption, and delayed seed- 
ling growth in peas. It is especially 
beneficial, however, to lettuce seed, 
which in turn is injured somewhat by 
Arasan and Fermate. Zinc oxide is 
injurious to p)cas but is highly benefi- 
cial to seed of spinach and crucifers. 

Potato tubers often are treated be- 
fore planting. Fifteen or more diseases 
of potatoes may be transmitted in or 
on the tubers. Few of them are ame- 
nable to control by treating the tubers 
before planting. Scab, rhizoctonia or 
black scurf, and fusarium seed-piece 
decay respond to seed treatment if 
they arc sccd-bome. The principal 
treatments recommended arc hot 
formaldehyde dip, cold formaldehyde 
soak, hot mercuric chloride dip, cold 
mercuric chloride soak, yellow oxide 
of mercury dip, and the hydrochloric 
acid-mercuric dip. Several organic 
mercury fungicides also are used. 
Among them are Semesan Bel, Sano- 
secd, and Gorona P. D. 7, all of which 
are made specifically for treating 
potato seed pieces. B^eficial results 
have been obtained also from the use 
of Fcrmate, Semesan, Spergon, and 
Dithane. 



TRiATINO SEEDS TO EEEVENT DISEASES 


Sweetpotatoes used for planting are 
treated to prevent injury due to 
seed-borne black rot, scurf, and stem 
rot and soil-infesting pathogens as 
species of Pythium^ Rhizoctonia^ and 
Sclarotium. The standard treatment is 
one lo-minute dip in a i to 1,000 
mercuric chloride solution, or a dip 
in Semesan Bel solution (i pound to 
1 % gallons of water). Both are effec- 
tive but sometimes they delay or 
reduce the production of sprouts. 
Fairly good results without injury 
have been had with Spergon, Phygon, 
Fermate, Zerlate, Tersan, and Pura- 
tized N“5-E. 

Some vegetable diseases, caused by 
soil infestation, are partly or wholly 
prevented by applying fungicides to 
the soil, usually with the fertilizer. 
Clubroot of cabbage has been con- 
trolled by adding calomel to the soil 
along with fertilizer and hydrated 
lime. Onion smut has been controlled 
by applying sodium nitrite, calcium 
nitrite, potassium nitrite, or Fermate 
to the soil a few days before .sowing. 
Arasan, similarly applied, controls 
onion smut and damping-off. Phygon, 
applied to the soil in fertilizer, con- 
trols damping-off in eggplant, pepper, 
beet, cucumber, and tomato. Differ- 
ent formulations of Di thane, applied to 
the soil, are said to be effective against 
red stele in strawberries, downy mil- 
dew in lettuce, blight in peppers, bed 
diseases of mushrooms, and damping- 
off in peas. The material acts either as 
a soil disinfectant or as a therapeutic 
agent. 

Treating the seeds of ornamentals 
is a common practice. Semesan has 
been widely used for this purpose. 

The nonmercurial organics, such as 
Arasan and Spergon, also are satis- 
factory. 

Ornamentals grown from bulbs, 
corms, tubers, and roots also are 
benefited somewhat by the use of 
fungicides. Gladiolus corms, for ex- 
ample, are helped by a 15-minute dip 
in a solution of i pound of New Im- 
proved Ceresan in 50 gallons of water 


*43 

just before planting. The standard 
mercuric and mercurous chloride solu- 
tions also are used. Dipping the corms 
in slurries of Spergon, Fermate, or 
Dow 9“B after digging is beneficial. 

Tulip bulbs have not responded 
very well to treatment. Some fungi- 
cides have lowered the yield of bul^. 
Dipping bulbs in slurries of Spergon 
or Fermate has increased some yields. 
Narcissus bulbs may be dipped in a 
phenyl mercury acetate solution (i 
pound to 800 gallons) for 5 minutes, 
after digging in spring and again 
before planting in fall, to control 
fusarium basal rot. Arasan SFX, Dow 
9“B, and New Improved Ceresan also 
arc beneficial. 

Hormones in seed treatments have 
been tried often. Results have varied. 
Of 30 investigators whose work was 
reviewed, 10 reported beneficial re- 
sults from the use of growth-pro- 
moting substances on seeds. Twenty 
failed to obtain any benefits. Appar- 
ently the conditions under which hor- 
mones may or may not be beneficial 
in seed treatments arc not fully under- 
stood. 

Growth-promoting substances are 
used commercially to induce root 
formation in cuttings, prevent fruit 
drop in apple orchards, induce fruit 
formation without pollination in some 
plants, and to prevent sprouting in 
stored potato tubers. It seems reason- 
able that under proper conditions the 
materials may improve seed germina- 
tion and early growth of the seedlings. 
Definite and reliable recommenda- 
tions cannot be made until more 
extensive research has been carried 
out. 

Synergism and antagonism be- 
tween different fungicides, when mixed 
together, has been demonstrated often 
enough to restrain one from mixing 
fungicides with one another or with 
insecticides without knowing how they 
will affect each other and the seedis 
on which they are to be used. 

A few examples of the effects of such 



YEARBOOK OF AGRICULTURE 1953 


144 

mixing may be mentioned. The addi- 
tion of New Improved Ccresan to 
DDT reduced both the fungicidal 
action of Ceresan and the insecticidal 
action of DDT. Magnesium oxide, 
added to copper carbonate or to 
Spergon, reduced the beneficial effect 
of those materials on emergence in 
wheat and on smut control in sor- 
ghum. Magnesium oxide also reduced 
the fungicidal efficiency of cuprous 
oxide and of Dow g-B, but seemed to 
increase the fungicidal effectiveness of 
sulfur. Pyrophyliite containing 3 per- 
cent DDT when mixed with Dow g-B 
reduced the control of sorghum kernel 
smut from 0.3 percent to 40 percent, 
with 60 percent infection in the check. 
Copper compounds in general are 
reduced in effectiveness when mixed 
with materials high in protein. 

A good fungicide, prepared especial- 
ly for seed treatment, usually is a 
well-balanced combination of active 
ingredients and suitable diluents, per- 
haps with the addition of wetting 
and dispensing agents, dyes, and other 
materials in proper projX)rtion. The 
addition of other materials, such as 
insecticides or other, fungicides, may 
cause chemical reactions and the 
formation of compounds that are in- 
effective as fungicides or injurious to 
the seed. 

The labels on containers for fungi- 
cides used for dusting or spraying 
vegetation often mention the insecti- 
cides with which they are not to be 
used. Labels for seed- treatment fungi- 
cides, however, do not include such 
directions because, as a rule, those 
fungicides are not mixed with insecti- 
cides or other fungicides, l^hat may 
change, however, with the growing 
need for combatting insects that attack 
seeds after they have been planted. 
Experiments in New York showed 
that Arasan SFX mixed with chlor- 
dane, lindane, or aldrin and applied 
to lima beans prevented both seed rot 
caused by fungi and seed injury due to 
the seed-corn maggot. Mergamma, a 
treatment for cereal seed, contains 
phenyl mercury urea for the control 


of certain cereal diseases and benzene 
hexachloride for wireworm control. 
The number of these insecticidal- 
fungicidal seed-treatment combina- 
tions doubtless will increase, but their 
use in combination should follow 
careful chemical and biological ex- 
periments. 

Seed injury following treatment was 
common when the treatments were 
mostly copper sulfate solutions, for- 
maldehyde, or mercuric chloride, 
especially when planting was delayed 
after treatment. 

When copper carbonate dust began 
to be used to treat wheat, it was found 
that delayed planting after treatment 
caused no injury to the seed but actu-- 
ally protected it against rodents and 
insects in storage. The more volatile 
organic mercury treatments, however, 
occasionally lowered the viability of 
seed after storage periods of more than 
a few days, especially when the mois- 
ture content of the seed was high. 
Several factors govern the degree of 
such injury: The moisture content of 
the seed; tlic volatility of the fungicide 
and the rate at which it is applied; the 
length of the storage period; the tem- 
perature, humidity, and aeration dur- 
ing storage; the kind of seed (seed of 
some genera, species, or varieties are 
more susceptible to chemical injury 
than are those of others); and the 
condition of the seed coat (cracked, 
chipped, or broken seed coats are 
conducive to seed injury). 

If seed is to be stored for a while 
after treatment with a volatile fungi- 
cide, its moisture content should be 
relatively low — 13 percent or less for 
cereals — and a lighter rate of applica- 
tion used. Different portions of oats of 
12 percent moisture content were 
treated with New Improved Ceresan 
at 1/2 and 1/8 ounce per bushel and 
cither sown at once or stored for several 
wccks.The seed treated at the i /2-ounce 
rate yielded better when sown the day 
after treatment. The seed that got the 
lighter application yielded better when 



TREATING SEEDS TO PREVENT DISEASES 


sowing was delayed for several weeks 
after treatment. Several experiments 
proved that sound seed of wheat, oats, 
and barley of good quality and proper 
moisture content, treated with one of 
the better organic mercury disinfect- 
ants at the recommended rate and 
properly stored for a year, was not 
injured in viability but yielded better 
than did untreated seed similarly 
stored. Occasionally in the more hu- 
mid areas of the Southeast, treated 
seed is stored with a too-high moisture 
content and the poor viability is 
ascribed to the treatment. Subsequent 
tests often show that the viability of 
the untreated seed is equally poor. 

Pretreatment of seed sold by seed 
dealers has been advocated for years. 
Some large seed houses pretreat seed 
of some field crops, such as cereals, 
flax, cotton, sugar beets, peas, corn, 
broomcorn, and some forage crops, 
either as a general practice or on a 
buyer’s request. 

Pretreatment of seeds by all dealers 
would mean cheaper but more general 
and more effective seed treatment; 
fewer outbreaks of preventable dis- 
eases; less waste of chemicals; less 
need of storing large stocks of chem- 
icals in many places; more economical 
packaging, distribution, and use of 
seed-treatment chemicals; the use of 
the proper disinfectant at the proper 
rate for each type of seed; and many 
other advantages. 

Some objections to general pre- 
treatment of seed are valid enough. 
There is no general agreement as to 
what treatment is best for each kind 
of seed. Some buyers object to planting 
“poisoned” seed. Some persons might 
not realize that treated seed is some- 
times poisonous and they might suffer 
injury. I think, though, that all the 
advantages of pretreating seed out- 
weigh the objections. 

Continued advances in seed treat- 
ment doubtless vrill bring new and 
better fungicides, better apparatus, 
and improved procedures into use. 
Fungicidal materials that promise to 


H5 

be more effective but less costly and 
less poisonous and disagreeable are 
sought. Slurry treaters that are more 
accurate and less troublesome arc 
promised. A process that will fix the 
slurry fungicide to the seed and 
prevent its dusting off when the dried 
seed is handled will rid the slurry 
method of its chief shortcoming. 

The possibility of systemic fungicides 
and chemotherapeutic disease pre- 
vention has been suggested and has 
been demonstrated in a few instances. 
This might eventually lead to the 
prevention of such Nation-wide calam- 
ities as epidemics of stem rust of 
cereals. Such fungicides would be 
applied to the soil and, when taken 
up by the plant, would render it 
resistant or fatal to the fungus attack- 
ing it. The fact that a tiny amount 
(3 parts per million) of selenium in 
soil is fatal to aphids and spider mites 
feeding on plants grown in the treated 
soil should encourage the search for 
fungicides equally effective against 
fungus infection but not poisonous to 
humans and animals. Such fungicides 
would be a tremendous advance in 
our war against plant diseases. 

R. W. Leukel is a plant pathologist 
in the division of cereal crops and diseases 
at the Plant Industry Station^ Bellsville^ 
Md, He has been engaged in the study of 
the cause and control of cereal diseases since 
I gig and is the author of more than 50 
articles on the subject. 



Conidia and cociidiophores* 



146 


YEAKIOOK OF AOIICUlTUEt 1«S3 


Making Sure 
of Healthy 
Seed 


Erwin L, LeClerg 

The primary purpose of seed certifica- 
tion is to maintain and to make avail- 
able to our farmers crop seeds, tubers, 
or bulbs of good seeding value and 
true to name. The factors considered 
in determining good seeding value or 
quality include viability, content of 
weed seeds, and freedom from seed- 
borne pathogens or viruses. 

The production of certified seed of 
superior varieties of field and forage 
crops involves the cooperative effort of 
many State, Federal, and private 
agencies. Among them are State agri- 
cultural experiment stations and exten- 
sion services, State -departments of 
agriculture, State crop improvement 
associations, the International Crop 
Improvement Association, the seed 
trade, and the United States Depart- 
ment of Agriculture. The work started 
at a meeting in 1919 of representatives 
of the States and Canadian certifying 
agencies. 

The determination of eligibility of 
varieties for certification is the chief 
responsibility of the State agricultural 
experiment station in every State. 
Factors considered in determining the 
suitability of a crop variety for inclu- 
sion in a ccrtificd-sccd program in- 
clude yield, adaptation, and resistance 
to diseases and insects. 

The requirements for eligibility for 
certification vary as to type of crop. 
^Certification for such cereal crops as 
wheat, oats, or barley, because of 


their limited area of adaptation, is 
relatively simple and frequently is 
conducted on a local-area basis. But 
the distribution of good seed of peren- 
nial forage crops — alfalfa, red clover, 
the grasses — requires the effort of 
many persons in widely separated 
areas because most of such seed is not 
produced in the locality where it is 
used for hay and pasture scedings. 

The farmer may make application 
for inspection but is under no obliga- 
tion to do so. 

Varieties of field crops must be ap- 
proved by a State agricultural experi- 
ment station before they are eligible 
to be considered for certification. In 
general, only one variety of the same 
crop, for seed production, is permitted . 
on a farm except by prior approval of 
the certifying agency. 

Field inspection is an important 
phase of the certification procedure. 
The International Crop Improvement 
Association has established certain 
minimum field standards, which form 
the basis for the regulations adopted 
by State certifying agencies. The 
standards take into consideration the 
type of crop, the degree of isolation 
necessary to prevent cross-pollination, 
and the class of seed produced. 

Four classes of seed are recognized 
in seed certification of field and forage 
crops: Breeder seed, foundation seed, 
registered seed, and certified seed. The 
International Crop Improvement As- 
sociation defines them thus: 

“Breeder seed is seed or vegetative 
propagating material directly con- 
trolled by the originating, or in certain 
cases the sponsoring plant breeder or 
institution, and which provides the 
source for the initial and recurring 
increase of foundation seed. 

“Foundation seed shall be seed stocks 
that are so handled as to most nearly 
maintain specific genetic identity and 
purity and that may be designated or 
distributed by an agricultural experi- 
ment station. Production must be 
carefully supervised or approved by 
representatives of an agricultural ex- 
periment station. Foundation seed 



MAKING SURE Of HEALTHY SEED 


shall be the source of all other certified 
seed classes, either directly or through 
registered seed. 

“Registered seed shall be the prog- 
eny of foundation or registered seed 
that is so handled as to maintain satis- 
factory genetic identity and purity and 
that has been^pproved and certified 
by the certifying agency. This class of 
seed should be of a quality suitable for 
the production of certified seed. 

“Certified seed shall be the progeny 
of foundation, registered, or certified 
seed that is so handled as to maintain 
satisfactory genetic identity and purity 
and that has been approv^ and certi- 
fied by the certifying agency.” 

State and Federal plant breeders for 
years have been developing sujjerior 
varieties of forage crops. For a long 
time, however, less than i percent of 
the legume and grass seed produced 
in the United States was of improved 
varieties. 

The great demand for seed of the 
newer varieties in forage-producing 
areas meant the draining away of the 
early generations of seed stocks, which 
should have been used to increase seed. 
The small supply of breeder seed has 
been the principal limiting factor in 
production of adequate supplies of 
foundation seed of some varieties of 
legumes and grasses. But, equally im- 
portant, no procedure for maintaining 
and distributing foundation seed of 
forage crops on a Nation-wide basis 
was available. 

The Foundation Seed Project was 
initiated in 1948 to set up the organiza- 
tion and financial procedure whereby 
foundation seed stocks of improved vari- 
eties of legumes and grasses could be 
rapidly pr^uced from breeder seed and 
distributed. Methods necessary to main- 
tain genetic purity of the varieties were 
devised. It is a cooperative effort uti- 
lizing the facilities of State and Federal 
agencies and the commercial seed 
trade. In 1952, 34 States participated. 
The cooperating agencies include State 
experiment stations. State extension 
services^ State seed certifying agencies, 
State foundation seed organizations. 


147 

the International Crop Improvement 
Association, the American Seed Trade 
Association, and the Department of 
Agriculture. 

The operational phases of the pro- 
gram are concerned with producing, 
assembling, distributing, and limited 
stockpiling of breeder and foundation 
seed. The coordination of those func- 
tions is the responsibility of the Bureau 
of Plant Industry, Soils, and Agri- 
cultural Engineering of the Depart- 
ment of Agriculture. Financial assist- 
ance for contracting for the mainte- 
nance of adequate reserves of breeder 
and foundation seed stocks is provided 
by the Grain Branch of the Production 
and Marketing Administration, repre- 
senting the Commodity Credit Cor- 
poration of the Department of Agri- 
culture. Each State is represented by 
a State foundation seed representative, 
who is responsible for initiating the 
foundation seed work and handling 
operations in his State. 

Direction of the program is the 
responsibility of a 16-man advisory 
group. Its members serve without pay. 
It is known as the Planning Con- 
ference and includes two representa- 
tives of each of these organizations: 
Northeastern Experiment Station Re- 
gion, Southern Experiment Station 
Region, North Central Experiment 
Station Region, Western Experiment 
Station Region, International Crop 
Improvement Association, American 
Seed Trade Association, Production 
and Marketing Administration, and 
Bureau of Plant Industry, Soils, and 
Agricultural Engineering. 

A superior variety developed by a 
plant breeder is tested for regional 
adaptation under the supervision of 
the Regional Forage Crops Technical 
Committee. A new variety found to 
be good in part or all of a region is 
then recommended by this technical 
committee to the Planning Conference, 
for inclusion in the Project. 

The Planning Conference deter- 
mines the areas for foundation seed 
production for each new variety, 
estimates the requirements for breeder 



YKAKBOOK OF AORICULTURE 1953 


148 

and foundation seed stocks, and co- 
operates with State workers in the 
areas of production in the develop- 
ment of the seed-increase program. 

The breeders’ seed of a new variety 
is assigned by the Planning Conference 
to State foundation seed representa- 
tives in the States where it has been 
determined that foundation seed is 
to be produced. All foundation seed 
is grown under contract with indi- 
vidual growers, who are selected by 
the State foundation seed representa- 
tive. Because the number of growers 
needed is small, only the most depend- 
able growers, located in an area with 
favorable soil and climate, arc chosen. 

The foundation seed of all varieties 
included in the program is allocated 
by the Planning Conference to State 
foundation seed representatives for 
planting to produce registered seed. 
The registered seed is handled by com- 
mercial seedsmen, who sell it to grow- 
ers for the production of certified seed. 
The certified seed, which is subse- 
quently used by farmers for forage 
plantings, is distributed through regu- 
lar seed trade channels. 

The 12 varieties included in the 
program in 1952 were Atlantic, Buf- 
falo, Narragansett, and Ranger alfal- 
fas; Kenland red clover; Tift Sudan- 
grass; Climax lespedeza; and five 
blend lines of Midland red clover. 

The 27 million pounds of certified 
seed of Ranger alfalfa available for 
1953 planting was the largest amount 
ever produced of an improved alfalfa 
variety. It was almost double the 
amount farmers had the previous 
year. The market had ahnost twice as 
much Buffalo alfalfa seed in 1952 as 
was available in 1951. The supply of 
certified Atlantic alfalfa for hay and 
pasture seedings had increased from 
150,818 pounds in 1950 to 1,511,000 
pounds in 1952. Narragansett alfalfa 
was added to the list of varieties in the 
Project in 1951. Yet it was possible to 
produce more than 5,000 pounds of 
certified seed the first year in addition 
to increasing breeder and foundation 
seed supplies. 


Enough stock seed of Kenland red 
clover was distributed to make avail- 
able 2 1 5,000 pounds of certified seed in 
1950, the first large-volume produc- 
tion. Yet in 1952, only 2 years later, 
there was a tenfold increase in certified 
seed production, which meant farmers 
had 2,000,000 pounds o^planting. The 
increase of Tift Sudangrass has been 
comparable. 

The potato tuber is subject to 
many diseases caused by fungus and 
bacterial pathogens and viruses. Dis- 
ease-free seed is important. 

The quality of seed potatoes has 
greatly improved since 1925 through 
the elimination of tuber-transmitted 
virus diseases by roguing, tuber-unit 
planting, tuber indexing, early harvest 
and pulling or killing vines; the use of 
winter field test plots; the use of im- 
mune or resistant varieties; the en- 
forcement of high standards of certi- 
fication; the production and use of 
better foundation seed; and the pro- 
duction of certified seed. 

Many tuber-transmitted diseases of 
potatoes, like mosaic, curly dwarf, 
spindle tuber, leaf roll, and blackleg 
{Erwinia atroseptica), cannot be con- 
trolled by spraying or dusting. The 
only way is to remove the diseased 
plants from the field, a procedure that 
is commonly termed roguing. Seed 
pieces, tops, and tubers are removed 
sufficiently far from the field and de- 
stroyed to prevent winged aphid vec- 
tors from migrating from the rogued 
plants or parts back to the potato field. 
Roguing usually commences when the 
plants are 4 to 6 inches above the 
ground. The first roguing Is followed 
by roguing at weekly or 10-day inter- 
vals throughout the season. 

Tuber-unit planting and tuber in- 
dexing have been generally used as an 
aid in the detection and elimination 
of virus diseases in the production of 
high-quality seed stocks. Tuber-unit 
planting, first practiced in 1908, is 
commonly used for seed plots and the 
production of foundation seed. The 



MAKING SUKf Of HEALTHY SEED 


method consists of planting all the 
seed pieces (usually four) cut from a 
single tuber one after another in the 
row, with a space separating them 
from the sets of the next tuber. Ready 
and accurate recognition of weak or 
diseased units thus is possible. All 
plants from the same tuber are re- 
moved if any one of them is weak or 
shows symptoms of a virus disease. 

Tuber indexing, first described by 
F. M. Blodgett and Karl H. Fernow 
at Cornell University in 1921, has been 
generally adopted in se^-producing 
areas. It consists of removing a small 
piece bearing a single eye from each 
tuber and planting it in the greenhouse 
in winter. Only the tubers whose seed 
pieces produced normal and healthy 
plants are retained for field increase 
the following spring. 

Foundation seed plots a mile away 
from other potato fields can become 
infected with virus diseases trans- 
mitted by winged aphids. As the 
growing season progresses, insect vec- 
tors of virus diseases may increase, 
and infection, contracted by the tops 
during the current season, is more 
likely to have reached the tubers in 
late- than in early-harvested stock. 
Healthy plants harvested early show 
a lower percentage of virus diseases 
than do healthy plants harvested late 
in the season. Early harvesting of seed 
plots or foundation seed is now a 
common practice in some potato seed 
producing areas. 

Hand pulling was one of the first 
methods employed to destroy potato 
vines in an effort to produce better 
disease-free seed. It soon became 
apparent that the method was imprac- 
tical because of the great cost and the 
high labor requirement. 

Flame burning destroyed the leaves 
but left stems standing, from which 
new growth was produced in late- 
maturing varieties. A similar situ- 
ation occurred when the foliage was 
mascerated with a rotobeater machine. 

The use chemicals to kill the 
vines has become a common practice 
in many of the potato seed pr^ucing 


149 

areas. It helps reduce the spread of 
virus diseases, prevent infection of 
tubers by the late blight fungus 
{Phytopkthora injestans)^ complete har- 
vest before freezing weather, regulate 
the size of seed tubers, and reduce 
skinning and bruising of tubers. Late- 
maturing varieties have been found 
to be more difficult to kill than early- 
maturing ones. Vascular discoloration 
of the tuber frequently occurs from 
the use of chemicals to kill vines. 

More work needs to be done to de- 
termine the factors that cause the dis- 
coloration. 

Because some weather conditions 
mask symptoms of some tuber-trans- 
mitted virus diseases, detection is 
sometimes difficult in the northern 
areas. The diseases can be detected 
when the plants are grown under field 
conditions in winter in some of the 
Southern States and California. Thus 
it is now mandatory to test all founda- 
tion seed (and much of the certified 
seed) in winter test plots. Through 
this means some northern growers of 
foundation seed have been able to 
maintain relatively disease-free stocks. 
The winter tests of samples from fields 
of potatoes grown for certification 
are completed in time so that the 
data may be used by certification 
officials of the seed-producing States. 
The information also helps growers 
of certified seed to keep from planting 
inferior stock. 

It is generally agreed that very little 
progress would have been made in 
controlling mosaic and other tuber- 
transmitted virus diseases if winter 
tests had not been established for 
guidance in the program of production 
of good foundation and certified 
seed. 

The production of the good, new, 
resistant varieties of potatoes is largely 
due to the accomplishments of the 
National Potato Breeding Program, 
which began in 1929. It is a Nation- 
wide program in which State agri- 
cultural experiment stations and the 
Department of Agriculture cooperate. 

The Katahdin variety was the first 



YEAtBOOK OF AGRICULTURE 1959 


150 

distributed under this program. It has 
resistance to mild mosaic and some 
resistance to leaf roll. It is immune to 
net necrosis. Nearly 1 3 million bushels 
of certified Katahdin seed were grown 
in the United States in 1952. In 1952 
it represented more than 30 percent of 
all certified seed potatoes and led all 
other varieties. Since then, 42 addi- 
tional varieties have been introduced, 
some of which have replaced older 
varieties to a significant extent. Not 
all are resistant to any of the major 
potato diseases but were relea^ 
because of their reported superior 
horticultural characters. 

Potato seed certification dates from 
1914 and the work of Professor J, G. 
Milward of the University of Wiscon- 
sin. It was the object of the work then, 
as now, to put upon the market the 
best seed that could be produced. 
Certification is recognized as a con- 
structive preventive measure in tuber- 
borne diseases. The industry has ex- 
panded greatly. The average produc- 
tion of certified seed potatoes in the 
United States, for the years between 
1949 and 1952, v/as 44.7 million 
bushels. 

The enforcement of certification 
standards is the responsibility of the 
colleges of agriculture and State de- 
partments of agriculture in most 
States. In Nebraska, Utah, and South 
Dakota, the work has been conducted 
by growers’ organizations. 

Certified potato seed is grown under 
a system of inspection. The plants are 
inspected twice in the field during the 
growing season. The tubers are in- 
spected in the bin after they are dug. 
The first field inspection is usually 
made early so as to identify and rogue 
the diseased plants. The second in- 
spection is made in the period between 
the time of blossoming and just before 
the vines mature. For each inspection 
a maximum percentage of affected 
plants or tubers for each disease is 
allowed. This disease tolerance varies 
for dise<ise and by certifying States 
but is usually between i and 5 percent. 


Each State has jurisdiction over its 
own certification work and sets the tol- 
erances permitted for various diseases. 
For example, the tolerances allowed for 
various diseases and varietal mixtures 
by Maine arc (the figures are percent- 
ages allowed in first inspection and 
second inspection, respectively): Leaf 
roll, 2 and i ; mosaic, 3 and 2; spindle 
tuber, 2 and 2; yellow dwarf, 0.5 and 
0.5; total virus diseases, 5 and 3; 
blackleg, 2 and 1 ; wilt, 2 and i ; bac- 
terial ring rot, o and o; total for all 
diseases, 6 and 4; giant hills, 1 (second 
inspection); varietal mixtures, i and 
0.25. 

The production of certified seed 
potatoes depends upon the quality of 
foundation seed stocks from which 
they arc grown. All plantings produced 
for certification must be grown from 
foundation stock of the best quality 
and should be thoroughly rogued. Im- 
provement of foundation seed stock 
through field roguing permits the re- 
moval of diseased, we^, off- type, or 
varietal-mixture plants during the 
growing season. In order to insure 
freedom from virus diseases, the foun- 
dation seed is produced in areas iso- 
lated from other potatoes and free 
from disease-transmission insects. A 
comparatively small number of tubers 
indexed one year and planted in the 
field the next spring will give a siz- 
able increase in seed stock for the fol- 
lowing year. After one season’s in- 
crease, a sufficient quantity would 
then be produced so that the grower 
could replace his old stock entirely 
with disease-free potatoes for the pro- 
duction of certified seed. 

The most recent development is the 
operation of seed-source farms by the 
certifying agencies. The farms (in 
Maine and some other States) arc oper- 
ated to produce seed stocks for growers 
of foundation seed. Seed is carefully 
grown in isolated areas and is tested 
in winter field test plots. Seed thus 
produced is released to selected grow- 
ers for the production of foundation 
seed. The varieties grown on the farms 
are planted by tuber-unit methods and 



MAKING SURE OF HEALTHY SEED 


carefully rogued by representatives of 
the certification agencies. 

Vegetable seed production areas 
have been shifted to areas where the 
weather is less favorable to the devel- 
opment of plant diseases. Such a shift 
is possible with crop-seed production 
because the total acreage of a particu- 
lar crop is usually relatively small in 
proportion to the crop for which the 
seed Ls used. 

Anthracnose {Colletotrichum lifidemu~ 
thianum) and the bacterial blights 
{Xanthomonas phaseoli and Pseudomonas 
phaseolicola) are three major diseases 
of beans in the United States. They 
are wet-weather diseases, and their 
spread and development depend to a 
great degree on the presence of high 
moisture and suitable temperatures. 

Before 1925, the production of bean 
seed was largely limited to the New 
England-Ncw' York area and Michi- 
gan. In those regions the weather 
conditions are generally favorable for 
the development of the three diseases. 
Between 1916 and 1919, losses in 
some of these localities were as much 
as 25 percent. For many subsequent 
years the diseases were widespread 
and destructis'e wherever beans were 
grown. 

Since the causal organisms of the 
diseases are disseminated chiefly with 
the seed and it is imperative to have 
clean seed for planting, attention was 
centered on seed production in the 
West, where weather is unfavorable 
to the development of the diseases. 
Such areas were found in Idaho and 
California. Those centers of certified 
seed production have elevations of 
2,000 to 3,000 feet, exceptionally low 
humidity, few showers, and little hail 
during the growing season. As a conse- 
quence of producing certified bean 
seed there, the three diseases have 
ceased to be the limiting factors in the 
production of the crop. 

At one time pea seed for gardens 
and canning was grown chiefly in 
the Northeastern States. Since 1925 or 
so, the center of production has lifted 


151 

to the irrigated and more arid parts 
of some of the Western States. Pea 
seed for these purposes is now pro- 
duced in the Snake River Valley and 
the Twin Falls areas of Idaho, near 
Bozeman, ‘Mont., the Palouse section 
of northern Idaho, and eastern Wash- 
ington. This shift was made in order 
to produce seed free of bacterial 
blight (Pseudomonas pisi) and leaf and 
pod spot (Ascochyia species). 

The growing of seed stocks of cab- 
bage, turnip, rutabaga, and cauli- 
flower used to be limited to the Mid- 
west and East. More recently the 
ravages of blackleg (Phoma lingam) 
and black rot (Xanthomonas campestris) 
have caused the shifting of the seed- 
producing areas for these crop.s to 
localities along the Pacific coast. 
Because of the low rainfall in those 
areas during the time the seedling 
plants are growing in seedbeds, the 
two diseases do not become estab- 
lished. Hence a crop of disease-free 
seed can be produced from the seed- 
lings, which are sub.sequenily trans- 
planted to clean fields. 

Most of the cauliflow’cr seed pro- 
duced in the United States is grown 
in the coastal valleys of California. 

In those areas, the seed has Ix'.en 
relatively free from both blackleg 
and black rot. 

The use of resist.aNT varieties, if 
available, is the most effective w‘ay of 
controlling the seed-boine organisms. 
That subject is discussed in the section 
that begins on page 165. 

Erwtn L. LeClerg is a research 
coordinator in the office of the Administra'^ 
tor of the Agricultural Research Adminis- 
tration, He is charged with coordinating 
parts of the research program of the De- 
partment that have to do with sugar crops, 
dry beans and peas, seeds, weeds, forage 
crops, pastures and ranges, pesticides and 
insecticides and related equipment. He holds 
degrees from Colorado Agricultural and 
Mechanical College, Iowa State College, 
and the University of Minnesota. He joined 
the Department in i^jo. 



*52 


YCAIBOOK or AOIICUITURI 1*S3 


How Nurseries 
Get Virus-free 
Fruit Stock 


L, C. Cochran^ Earle C. Blodgetl, 
J. Duain Aloore, K. G, Parker 

More than 40 virus diseases affect 
stone fruits. Others attack pome, 
citrus, avocado, fig, and other fruits. 
No major fruit crop is free from 
virus diseases. Some virus diseases have 
caused the destruction of orchards 
throughout communities and others 
have ruined orchards in large areas. 
Some, more insidious, do not kill trees 
but take an annual toll by reducing 
the yield and quality of the crops. Still 
others produce only mild effects and 
are important chiefly because they 
complicate the question of control. 

Part of the geographic occurrence 
of most of the virus diseases affecting 
stone fruit is traceable to distribution 
in infected nursery stock. Most fruit 
trees now are grown from nursery 
stock produced by budding or grafting 
the desired variety onto seedlings or 
rootstocks propagated vegetatively 
from cuttings. If the variety or the 
rootstock is infected with a virus, the 
resulting nursery trees usually will be 
infected. If the nursery is located near 
infected trees, viruses may spread nat- 
urally into nursery stock during the 
growing season. When diseased nurs- 
ery stock is planted in a district where 
the disease was not previously present, 
spread may take place toother trees and 
the disease soon becomes established. 

The question of nursery improvement 
is complex. The many factors that 
are involved vary among areas because 
different diseases are present, different 


varieties of fruit are grown, and differ- 
ent conditions exist in each. It seems 
impossible to devise a program with 
provisions that would be entirely ap- 
plicable to all areas, although some 
general procedures can be formulated* 

Because some virus diseases of stone 
fruits are known to have been carried 
in the nursery stock, a logical starting 
point for improvement is the use of 
procedures to eliminate them from 
scion and budwood sources and from 
rootstock seed sources. Growers of such 
sources then could be issued certifi- 
cates indicating the standards that 
have been met. Certification has value 
only w'hen it refers to specified definite 
standards. 

Any plan for the production of cer- 
tified fruit tree nursery stock could 
well make use of the same principles 
and procedures developed for certified 
seed potatoes: Establishing disease-free 
foundation stocks true to variety; in- 
creasing the stocks in the field under 
rigid inspection and roguing; passing 
the stocks for certification if the num- 
ber of diseased 01 off-type plants is 
maintained at or below a standard, 
w'hich has l>een determined by prac- 
tice to be necessary to insure high 
yields and good quality; and super- 
vising sales to maintain the identity 
of the certified stock. 

Considerable progress has been 
made toward nursery improvement in 
the more important fruit-growing 
States. The program has been mostly 
voluntary. The approach has been 
from different angles. Unknown and 
variable factors have prevented the 
formulation of any uniform procedure 
usable, in all States. 

In most of the States, the first step 
has been to inspect the orchard trees 
desired as a source of budwood and 
the trees adjacent to them. If no symp- 
toms of virus or viruslike diseases are 
found, the nurseryman who gets buds 
from them may obtain a certificate 
that his trees were propagated from 
sources that had been inspe<;tcd and 
found visibly free of virus diseases. 
The procedure has helped reduce the 



HOW NUISEIIES GET VIIUS-FEEE FlUIT STOCK 


prevalence of such vims diseases as the 
peach yellows group, peach wart, and 
certain cherry diseases, which are gen- 
erally expressed on all the horticultural 
varieties of the aflected host. It has also 
helped in the elimination of cherry 
diseases like mottle leaf, twisted leaf, 
msty mottle, the necrotic rusty mottle; 
apricot ring pox; the psorosis of citrus; 
and other diseases that damage some 
fruit varieties but are only mcagerly 
expressed on others. Orchard inspec- 
tion has also materially helped in the 
elimination of certain viruslike non- 
transmissible but bud-perpetuated dis- 
orders, such as sweet cherry crinkle 
leaf, sweet cherry deep suture, almond 
bud failure, and Italian Prune leaf 
spot and sparse leaf, and has assisted 
in the selection of fruitful types time to 
variety. 

The usual procedure has been for 
the nurser>'man to apply for the service 
by a given date to the Stale depart- 
ment of agriculture in his State. The 
trees then are inspected in the proper 
season and given some sort of identi- 
fying designation. Standards have to 
be set, such as minimum age of the 
trees to be used and the distance from 
the nearest diseased trees. Because 
most of the diseases spread in orchards, 
any certificate based on orchard in- 
spection is good only for the year in 
which the inspection is made and new 
inspections must be made each year. 

Orchard inspection alone is not 
enough to determine the presence of 
all the viruses that affect fruit trees. 
Some viruses that arc destructive to 
one variety may exist in another with- 
out symptoms. Buds from such infected 
but symptomless trees produce similar 
infected nursery trees, which carry the 
vims to orchard locations where the 
trees are planted. The mottle leaf 
virus mins the Napoleon (Royal Ann) 
and Bing varieties of cherries, but may 
cause few symptoms or none on Lam- 
bert. 

Environment, such as high or low 
temperatures, influences the expression 
of symptoms of some disease.s. Symp- 
toms leaf yellowing of the sour 


*53 

cherry yellow^s disease arc expressed 
on sour cherry trees and damage is 
accentuated in areas W'here tempera- 
tures following petal fall are relatively 
low, but no leaf symptoms occur in 
areas where the temperatures arc 
higher. Nursery stock propagated in 
warmer climates from vigorous-ap- 
pearing trees can very innocently 
carry the sour cherry yellows virus and 
result in serious losses if planted in such 
areas as those near the Great Lakes, 
where summer growing temperatures 
are low. 

The western X-disca.se vims, con- 
versely, may not produce symptoms, 
especially on sweet cherries growing on 
mazzard rootstock in areas of high 
elevation where temperatures arc low. 
Some virus diseases have long incuba- 
tion periods; hence, if buds arc cut 
from orchard trees in the early stages 
of infection before symptoms appear, 
they may carry the viru.s to nursery 
slock. The ring spot virus produces 
symptoms only during the acute or 
initial stages of infection on many 
fruit tree hosts, yet buds taken from 
trees in the chronic stages and showing 
no symptoms carry the virus. 

The pre.scnce of viruses in symptoin- 
Icss trees is determined by indexing 
them on varieties that express symp- 
toms. That commonly is done by bud- 
ding healthy nursery trees of a suscep- 
tible symplom-exprc:)Sing variety with 
buds from the suspect trees. Certain 
varieties and species arc known to 
produce consistent and characteristic 
symptoms when infected with particu- 
lar viruses. By use of a combination of 
such hosts in index procedures, a fruit 
tree can be tested for the pre.scnce of 
any of the known virusc.<l. In order to 
keep the number of necessary hosts to 
a minimum, hosts that will express 
and differentiate a large number of the 
viruses may be used. Here is a list of 
index hosts and the diseases which each 
may serve to diagnose: 

Peach Elberta: Peach yellows, little 
peach, red suture, |>cach rosette, 
rosette mosaic, phony, peach mosaic, 
X-discase, western X-disease, yellow 



YfAltBOOK OF AGilCULTURE 1953 


154 

bud mosaic, wart, peach motile, peach 
necrotic leaf spot, asteroid spot, gold- 
en-net, peach calico, peach blotch. 

Peach. J. H. Hale: Ring spot, willow 
twig. 

Peach. Muir: Muir peach dwarf. 

Peach. Seedlings (open-pollinated 
seedlings of Lovell and Halehaven have 
been used): Necrotic ring spot, sour 
cherry yellows. 

Sour cherry. Montmorency: Sour 
cherry yellows, green ring mottle, 
necrotic ring spot, pink fruit, peach 
mottle. 

Sour cherry. On mahaleb: Western 
X-di.sease will and decline. 

Sweet cherry. Bing: Buckskin, al- 
bino, mottle leaf, rusty mottle, mild 
rusty mottle, rasp leaf, twisted leaf, 
latter leaf, small bitter cherry, western 
X little cherry, peach mottle. 

Sweet cherry. Royal Ann: Black 
canker, cherry rugose mosaic, pinto 
leaf. 

Sweet cherry. Lambert: Necrotic 
rusty mottle, little cherry, small bitter 
cherry, Lambert mottle, Utah Dixie 
rusty mottle. 

Prunus sertulala vars. Shiro/ugen: Ring 
spot. 

Prunus serrulata vars. Kwanzan: Other 
latents, rough bark. 

Plum. Italian Prune: Prune dwarf. 

Plum. Shiro: Line pattern. 

Plum. French Prune: Prune diamond 
canker. 

Plum. Standard prune: Standard 
prune constricting mosaic. 

Plum. Santa Rosa: Plum white spot. 

Apricot. Tilton: Ring pox. 

Almond. Nonpareil: Drake almond 
bud failure. 

It may not be necessary to index 
bud wood sources in all areas on all of 
those hosts. For example, our evidence 
indicates that some of the cherry vi- 
ruses are not present in peaches in 
some sections where only peaches are 
grown. Also, mo.st peach virus diseases 
(or at least most of those that seriously 
damage peaches) affect all varieties of 
peaches similarly, and their presence 
generally is determined easily by or- 
chard inspection. But cherries appear 


to be more commonly affected by 
viruses than peaches. Some viruses are 
ruinous on one variety of cherry but 
may infect another with only meager 
or no symptoms; index procedure to 
find virus-free trees therefore is needed 
for cherries more than for peaches. 

The question of how much indexing 
should be done depends on what dis- 
eases are present in the area, the des- 
tination of the nursery stock, and the 
variety of the host. 

Stone-fruit clones completely free of 
all viruses are difficult to achieve for a 
number of reasons. The complete host 
range of many of the stone-fruit viruses 
has not yet been determined. Com- 
pletely .satisfactory index hosts for all 
of the stone fruits have not been de- 
termined, particularly because of the 
variability of reaction caused by dif- 
ferent forms of certain viruses. Some 
stone fruits are nearly universally in- 
fected, particularly with some of the 
latent viruses. Certain viruses (sour 
cherry yellows and ring spot) are 
transmitted through seeds. As insect 
vectors are known for only a few of 
the diseases, it is not known what 
measures are necessary to protect 
healthy stock from outside infection. 

The problem of obtaining virus-free 
stocks by indexing procedure is com- 
plex. Index hosts have to be found 
that will serve with certainty to indi- 
cate the presence of a given virus in all 
its forms. The search for such hosts has 
been complicated by the fact that 
stocks of certain of those used w^ere 
already infected. Orchard trees being 
indexed often arc infected by more 
than one virus and therefore give con- 
fusing results. The long incubation 
period of some virus diseases, such as 
diamond canker of French Prune and 
willow twig of peach, makes the pro- 
cedure slow and costly. 

Some of the difficulties are exempli- 
fied by the efforts to get sour cherries 
free of the sour cherry yellows virus. 
The climate around the Great Lakes 
appears to be well suited to sour cher- 
ries, and most of the sour cherry or- 
chards on the United States arc there. 



HOW NURSERIES GET VIRUS-FREE FRUIT STOCK 


The climate also favors development 
of the sour cherry yellows disease, the 
cause of serious losses. Infected trees or 
nursery stocks grown in warmer cli- 
mates may not be seriously affected 
and may not show leaf symptoms. A 
simple procedure for testing the sour 
cherry orchard trees for the presence 
of the sour cherry yellows virus or free- 
dom from it would be to grow proge- 
nies located in an area where, if the 
virus were present in the progenies, 
symptoms would be sure to develop on 
them. Trees of other species could be 
correspondingly tested by placing buds 
from them into healthy sour cherry 
nursery trees. The chief objection to 
this procedure is that the nursery trees 
propagated from diseased trees, or 
healthy nursery trees infected by inoc- 
ulation from diseased trees, sometimes 
take 2 years to develop symptoms; 
thus it takes a long time to get results. 
Some difficulty has been experienced 
in obtaining and maintaining disease- 
free indicator trees. Also, sour cherry 
does not expre.ss symptoms of many of 
the other viruses which may be present, 
thus necessitating further indexing on 
other hosts. 

A high percentage of sour cherry trees 
is infected with the ring spot virus. 
Build-up of this virus has taken place 
over the years by indiscriniinnie prop- 
agation frf»m infected trees, in which 
the virus had become latent; by propa- 
gation on infected seedling rootstocks, 
infected by passage of the virus through 
seeds; and by orchard spread. 

Ring .spot is much more prevalent 
than sour cherry yellows; in fact, ail 
cultures of sour cherry yellows appear 
to contain the ring spot virus. 

The universal occurrence of ring spot 
with yellows might indicate that yel- 
lows is the expression of the combined 
effect of two or more viruses, of which 
ring spot is one. Ring spot is known to 
exist without yellows and may be due 
to a single virus that is generally a con- 
taminant of yellows. Any index proced- 
ure for sour cherry yellows would neces- 
sarily have to take ring spot into ac- 
count. 


155 

In Michigan, index procedure for 
sour cherries has l)cen developed with 
peach seedlings as indicator plants. 
Halehaven peach seedlings grown dur- 
ing the current season arc budded with 
cherry buds in late August. If ring spot 
alone is present in the sour cherry trees 
from which the peach seedlings were 
budded, grow'th of the seedlings the 
following spring is retarded, buds die 
on many of the branches, and some- 
times the branches die. Subsequent 
new peach growth from surviving buds 
assumes a normal appearance. 

If sour cherry yellows is present in 
the cherry trees, the inoculated peach 
trees will show the retarding and die- 
back characteristic of and caused by 
the ring spot virus l)ut in addition sub- 
sequ(‘nt peach grqjA’th from surviving 
buds })roduccs shoots with short inter- 
nodes and abnormally green leaves 
crowded into loose rosettes. If neither 
ring spot nor sour cherry yellows was 
present in the clierries, the peach seed- 
lings grow normally and should com- 
pare with un inoculated checks. 

"I'lie peach seedling technique is quick 
and inexpensive. It can be done on a 
large scale in many areas in the open 
field. Its shortcomings are that peach 
does not react well in the greenhouse 
and cannot be used out-of-doors in 
regions wliere peaches are subject to 
winter injury. In sonic instances, poss- 
sibly l)ccausc of virus forms or individ- 
ual seedling differences, peach docs not 
give clcar-ciit reactions. The possibility 
exists that the dwarfing leaction attrib- 
uted to yellows is due to a third virus, 
which is a contaminant commonly as- 
sociated with sour cherry yellows. 

In Wisconsin, indexing procedure 
has been developed by making use of 
the fact that ring spot is usually associ- 
ated with sour cherry yellows. Index- 
ing is done in one of two ways. Scions 
from the irecs are grafted on potted 
disease-free Montmorency cherry trees 
in the greenhouse and held at 70° F. 
for 3 to 4 weeks; if the orchard tree has 
ring spot, ring spot symptoms will de- 
velop on the leaves of the potted tree. 
The second way is to cut scions from 



Ijb VIA«100K OF AORICUlTURi 1943 


the orchard tree and hold them in cold 
storage. The tree is inoculated with a 
known culture of ring spot and ob- 
served for symptoms, if no symptoms 
develop, the tree is assumed to have 
had ring spot l^efore the inoculation 
and the scions in storage are discarded. 
If the tree develops ring spot symp- 
toms, it is assumed that it was not pre- 
viously infected and the scion wood in 
storage is used for propagation. By 
eliminating ring spot, it is reasoned 
that sour cherry yellows also is elimi- 
nated. Our experience thus far has sup- 
ported this conclusion. 

Use has been made in Oregon of 
two varieties of Prunus serrulate, Kwan- 
zan and Shirofugen, for indexing for 
the presence of ring spot and possibly 
other latent viruses. When buds 
carrying ring spot are inserted into 
arms of Shirofugen, the buds die 
without uniting, and gumming le- 
sions are formed around the bud 
insertion points. The virus apparently 
moves very slowly, because if the 
^branch is severed below the gumming 
lesion the virus is removed. By spac- 
ing index buds at intervals of 6 inches 
or lc.ss along a branch, a single Shiro- 
fugen tree can be used to index a 
large number of orchard trees. Trees 
that test negative on Shirofugen arc 
then tested on Kwanzan, because in 
a few instances a virus has been 
found that wiU not affect Shirofugen 
but will cause a reaction on Kwan- 
zan. Viruses usually spread rapidly 
through Kwan/an. A Kwanzan tree 
therefore can he used only for one 
test. 

Prunus tomeniosa, Manchu cherry, 
has been used in Iowa and is reported 
to be a more sensitive host for ring 
spot than Lovell peach seedlings. 
Manchu cherry was found unsatis- 
factory in California because of varia- 
bility among seedlings and its failure 
to give a reaction with forms of the 
ring spot virus which reacted on Hale 
peach. Results of tests in Washington 
indicate that Shirofugen is a much 
more sensitive test plant than Manchu 
cherry. 


More information is needed before 
the different inde.v hosts can be 
evaluated. Where sour cherries arc 
maintained as clones and do not 
show any symptoms of sour cherry 
yellows under growing conditions 
favorable for yellows, it can be pre- 
sumed with reasonable certainty that 
they arc free of yellows. Peach, sour 
cherry, and the Shirofugen and Kwan- 
zan varieties of oriental flowering 
cherry all appear to be of value in 
indexing for ring spot. Shirofugen 
can be grown in climates with in- 
sufficient chilling requirements for 
sour cherries and appears to be as 
sensitive to the ring spot virus as sour 
cherry or peach. 

Programs have been started in some 
States to develop certified foundation 
stocks. In some instances nursery- 
men were furnished budwood direct 
from orchard trees that had been 
determined by index methods to be 
free of virus. Tree performance was 
determined by observations directly 
on the orchard tree. In other in- 
stances progenies have been grown 
from indexed orchard trees and bud- 
wood has been supplied to the nurs- 
erymen from the progenies. In a few 
instances enough budwood for direct 
propagation of nursery stock has been 
supplied from progeny trees, but 
mostly nurserymen have increased 
their own foundation stocks to supply 
budwood sulTicicnt for their needs. 

Peach budwood heated at 122® F. 
for 5 minutes was used in Michigan 
on a sufficiently large scale to show 
that such treatment was practical 
for nursery procedure. Experiments 
earlier had shown that the treatment 
would eliminate viruses of the peach 
yellows group and X-disease. Certi- 
fication on the basis of inspection 
of orchard trees has been used satis- 
factorily in Michigan for avoiding 
the yellows group of diseases in 
nursery stock; the heat treatment 
adds assurance against any of these 
diseases getting through. 

A project has been undertaken in 
Washington by the State Department 



MOW NUtSIRfiS OfT VlftUS-FREI FRUIT STOCK 157 


of AgfricuJture to assemble commercial 
stone-fruit varieties free of the known 
virus and virusiike diseases and to 
grow them at an isolation station 
near Moxee. The station is about 8 
air miles from the closest commercial 
orchards. The water supply is such 
that the development of orchards 
nearby is unlikely. Wild species of 
pTunus arc not present in the locality. 
Cooperating in the project are re- 
search divisions of the United States 
Department of Agriculture and the 
Washington State Agricultural Experi- 
ment Station. The plan is to establish 
foundation stocks, true to variety and 
free from diseases, that can be supplied 
to nurserymen for increase. Nursery- 
men arc encouraged to use the bud- 
wood they obtain from Moxcc to 
establish their own blocks of mother 
trees, from which they can get buds 
for nursery propagation. 

The foundation stock-mother block 
procedure has several advantages over 
use of approved trees in orehards. 
Mother- block trees projierly isolated 
can be maintained with le.ss risk of 
becoming infected by natural spread. 
Pedigreed stocks, centrally located, 
can be given proper care and can 
more easily be checked for off- types or 
diseases. Centralization also makes for 
simplification of record keeping, 
especially as regards progeny per- 
formance and tracing troubles which 
may arise. 

It also has limitations. Standards 
may be hard to determine. It has 
not been established that certain 
viruses, especially certain of the latent 
ones, are sufficiently harmful to war- 
rant exclusion. New diseases arc 
continually being found, and it may 
be difficult to prevent some of these 
from getting into stocks. There will 
be a continuing demand for the new 
varieties and new strains of varieties, 
which will pose a continuous problem 
of what should be stocked. 

If the isolation station procedure is 
generally adopted, much expense can 
be saved by putting it on a regional 
basis. In the same way, procedure 


for interstate shipment of nursery 
stock could be simplified by inter- 
state agreement on the requirements 
for certification. 

The most serious virus diseases of 
citrus and pome fruits in most in- 
stances can be detected and avoided 
by orchard inspection. The psorosis 
disease of citrus occurs in all of the 
citrus-growing areas of the world. It 
was distributed in infected budwood 
before people knew it was caused by 
a virus. Infected trees do not commonly 
develop the spectacular scaling lesions 
and decline symptoms until they 
are 12 to 16 years old, yet the buds 
taken from them before the symptoms 
appear carry the virus. Exocortis, a 
disease that affects trifoliate orange 
rootstock, re.sulting in dwarfing the 
trees growing on it, can be carried 
by the top variety without symptoms 
if it is grown on other rootstocks. 
Stubborn disease, a third virus disease 
that reduces the vigor and fruitfulness 
of sweet orange, also requires several 
years for recognizable symptoms to 
develop in infected nursery stock. 

All these diseases can be avoided 
by selecting budwood from vigorous, 
healthy-appearing orchard trees old 
enough to show symptoms. For free- 
dom from the exocortis virus, bud- 
wood must be taken from trees grow- 
ing on trifoliate orange rootstock. 
The discovery that psorosis could be 
diagnosed on the basis of symptoms in 
young leaves has greatly simplified 
certification procedure. Sweet orange 
seedlings can be used to index any 
species of citrus on which the presence 
of psorosis cannot be determined by 
orchard inspection. 

None of the three citrus viruses we 
mentioned appears to have any means 
of natural spread in North America 
except by occasional natural root 
grafts between trees. Production of 
virus-free nursery stock is therefore 
an important and efficient way to 
control them. 

The contagious nature of the quick 
decline disease of citrus, the lack of 
distinctive symptoms, and the wide 



rCARBOOK OF AORICUlTURf 195 $ 


158 

occurrence of the causal virus in 
sweet orange on rootstocks other 
than sour orange make production of 
nursery stock free of the quick decline 
virus impractical as a control pro- 
cedure within infected areas. 

Several virus diseases affect pome 
fruits, but only one, stony pit of pear, 
has caused sufficient damage to merit 
selection of disease-free scion wood. 
Fruits of the Bose variety on diseased 
trees are variously misshapen and 
pitted. Tissue at the ba.se of the pits 
and around the core becomes hard and 
stony, making the fruit worthless. The 
di-sease can readily be recognized in 
the orchard just Ix'fore harvest on 
frviits of the Bose variety. Other varie- 
ties can be indexed by grafting healthy 
Bose on one arm. 

One rather .serious virus disease of 
avocado, sun blotch, is the cause of 
unfruitfulness and misshapen fruits. 
The expression of the cli.scase is erratic 
and no good indicator variety is known. 
The best method of avoiding the dis- 
ease seems to be l)y the use of f)ropa- 
gation material from trees shown liy 
progeny performance to be free of sun 
i3lotch. 7’he disease appears to have 
been spread chiefly in di.sea.sed nursery 
stock and is the cau.se of enough dam- 
age to w'arrant efiorts to avoid it. 

Elimination of viruses and vinislike 
disorders from nursery stock must pre- 
cede measures applied in the orchard 
for effective control. "I'o do this, .several 
general steps are ne.ce.ssary. Viru.scs 
and viruslike disorders, which cause 
obvious symptoms, can be avoided by 
use of bud wood from orchard trec.s 
that show no .symptoms, are fruitful, 
and true to type. Such trees preferably 
should not be in plantings where, con- 
tagious virus dise«j.se.s are pre.sent and 
should not in any case Ijc adjacent to 
virus- infected trees. Screening by in- 
dex procedure is necessary to avoid 
viruses that may be latent in orchard 
trees. When desirable trees are once 
determined free of virus, they should 
l>c propagated on virus-free rootstocks 
and grow'n under isolation where they 
can be maintained under observation 


and periodic testing to assure virus 
freedom and desirability of type. Such 
trees can serve as foundation material 
from which propagating materials can 
he supplied to nurserymen for estab- 
lishing mother tree blocks, which in 
turn supply budwood for nursery 
propagation. 

It is equally important that orchards 
producing seeds for growing rootstocks 
be virus-free and of desirable type. 

Specifications for indexing and isola- 
tion would necessarily vary' with dis- 
tricts, depending on the disea.scs pres- 
ent and the fruits grown, but effort 
should be made to devise provisions 
with enough uniformity to allow for 
interstate shipment. Growers should 
demand virus-free trees. Nurserymen 
need the cooperation and a.ssistance of 
rc.search, regulatory, and extension 
men, and growers. Nursery improve- 
ment j)rograms are under development 
in .several States and there is rea.son 
for optimism. 

L. C. Cochran is in charge uj investi- 
gations oj virus diseases of deciduous fruits 
in the Bureau of Plant Induslr}\ Soils^ and 
Agricultural Engineering . 

Earle C. Bi.odoett is located at the 
Irrigation Experiment StalioHy Prosser^ 
Wash.y and holds a joint position of plant 
pathologist with the Washington State De- 
partment of Agriculture and the Washington 
Agricultural Experiment Station, He is 
responsible for developing fruit tree founda- 
tion stocks and nursery improvement pro- 
cedures. 

J. Du AIN Moore is associate professor 
of plant pathology in the University of 
Wisconsin and is engaged in investigations 
of diseases of tree fruits., with particular 
interest in virus diseases of sour cherries. 
He is a native of Ijincaster County^ Pa.,, 
and holds degrees from Pennsylvania State 
College and the University of Wisconsin, 

K. G. Parker is professor of plant 
pathology at Cornell University. He has 
been investigating diseases of trees since 

and has been in charge of investiga- 
tions of virus diseases of tree fruits since 
ig 46 . He is a native of Indiana and holds 
degrees from DePauw and Cornell, 



THE INSPECTION OF IMFOETEO FIANTS 


The Inspection 
of Imported 
Plants 


Donald P, Limber, Paul R. Frink 

Federal plant quarantines have been 
in effect in the United States since 
1912. They do not prevent the impor- 
tations of large numbers of plants each 
year. The plants offered for introduc- 
tion must be examined to determine 
tliat they arc free from plant pests that 
are not present in our country or are 
not widely established here. This first 
examination at the ports of entry 
searches out all plant pests — insects, 
fungi, virus diseases, and nematodes. 

The Bureau of Entomology and 
Plant Quarantine is responsible for the 
pest-risk problem in the importation 
of plants. Two States, California and 
Florida, collaborate with the Federal 
Government at the ports within their 
borders. All States cooperate in the 
follow-up inspections that are given 
certain genera of plants when they arc 
grown in the field under post entry 
quarantine. 

The inspector’s basic tools are a 
hand lens and a microscope. He ex- 
amines the imported plant material to 
sec if any plant diseases are present 
and identify the ones he finds. On the 
identification depends the decision as 
to whether those plants should be 
rejected, treated, or released. If he 
cannot identify the disease, the. inspec- 
tor holds the shipment and refers a 
specimen to the Washington office for 
determination by specialists. 

The very nature of plant diseases 
makes it hard to enforce plant quaran- 
tines. Bacteria and the spores of the 


159 

higher fungi that spread the diseases 
are so minute that they are usually 
invisible without magnification unless 
massed in large numbers. Even when 
the spores have germinated and in- 
vaded the tissues of the plant, evidence 
of the developing disease ordinarily 
docs not appear at once. Leaves in- 
oculated with Celletotrichum cypripedii 
show the first symptoms in 15 or 16 
days. That is about the normal period 
for the incubation of many other dis- 
eases, but some may appear in as few as 
4 days and others only after a month 
or longer. 

Few fungus diseases can be eradi- 
cated when they arc present in living 
plants. Therefore the plants on which 
a new disease is found usually are 
rejected. There are some exceptions. 
Hot-water treatments for nematode 
diseases and a few fungi (such as the 
leaf smut of rice, Entyloma oryzae^ and 
the mint rust, Puccinia menthae) under 
favorable conditions may eliminate the 
parasites. Another example: The citrus 
seeds may carry the bacteria Xan- 
thornonas citri, w^hich cause the citrus 
canker. Citrus seeds immersed in a 
soluiion of I part peroxide of hydrogen 
to 2 parts of water for i o minutes arc 
completely cleansed of viable citrus 
canker bacteria. 

The plant quarantine inspector 
at Houston, Tex., made his customary 
examination of the stores of a cargo 
vessel from Japan. The fruits and 
vegetables in ship’s stores, though for 
use on the ship, may be a serious risk. 
The crew members may attempt to 
smuggle fruit ashore. Peelings and 
spoiled stores may be thrown into 
harbors and washed ashore. 

In the stores the inspector found 
five citrus fruits on whose rinds were 
numerous small, round, corky spots. 
The fruits were confiscated and de- 
stroyed, as citrus from Japan is pro- 
hibited entry into the United States. 

Samples of the rind bearing the 
spots were sent to a Bureau mycologist 
at Hoboken, N. J. All five fruits were 
infected with citrus canker, Xantho^ 



i6o 


YCAtSOOK Of AOIICULTURI 1*59 


monas airt\ a highly destructive disease 
of citrus and one that is believed to 
have been completely eradicated from 
the United States after long and ex- 
pensive effort. 

Citrus canker was found in passen- 
gers’ baggage and ships’ stores on lo 
occasions in a single month at the 
port of San Francisco — an illustration 
of the need for continuous vigilance. 

At the old inspection house in 
Washington, D. C., which stood on 
the corner of Constitution Avenue at 
Twelfth Street, an inspector was ex- 
amining some orchids from the wilds 
of Brazil. Some small patches of 
dusty, yellow material on a leaf caught 
his eye. 

It seemed harmless at first glance 
and resembled amorphous materials 
sometimes found on orchid leaves. 
Upon turning the leaf over, however, 
he saw that there was a yellowing of 
the tissue extending through the leaf. 
Other leaves with more numerous 
spots were found. Some of the leaves 
were dead. 

When sections were made, the dusty 
material proved to be the uredospores 
of the rust, Hmileia oncidii. All the 
infected leaves were removed and de- 
stroyed, The plants were then disin- 
fected by a dip in bordcaux mixture. 
Hemileia oncidii and other rusts of 
orchids, which have also been inter- 
cepted many times, are not known to 
be established in the United States. 

A LARGE SHIPMENT of lily-of-thc- 
valley pips (Convallaria majalis) arrived 
on the docks at New York in 1950, 
The pips originated near Hamburg, 
Germany. The inspectors at the port 
gave them the usual thorough exam- 
ination on the pier, as these plants 
are a known host of the stem and bulb 
nematode {Ditylenchus dipsaci). 

No Ditylenchus was found, but the 
roots carried some sandy soil. Soil is a 
serious hazard in quarantine enforce- 
ment. It may carry insects, particu- 
larly the larvae and pupal stages, 
various soil-borne fiingi, and plant 


nematodes. Imported plant material 
therefore must come with clean roots. 
The pips were ordered to be sent to 
the Hoboken Inspection House for 
cleaning. At Hoboken the washings 
from the roots were examined care- 
fully for plant pests, and finally proc- 
essed to recover any nematode cysts 
that might be present. 

The plant pathologist, looking down 
through the microscope at the debris 
Boating in the dish beneath the lens, 
observed a smooth, dark-red, spheri- 
cal body with a short neck, floating 
around with the debris. The whole 
object was less than i millimeter in 
diameter. Here was a truly dangerous 
immigrant — the cyst of the golden 
nematode (Heterodera rostochiensis) filled 
with living eggs. 

Further search revealed that it was 
not a lone cyst, but that the whole 
shipment of pips was heavily contami- 
nated with golden nematode cysts, 
by hundreds at least, and possibly by 
thousands. Each cyst contained 10 to 
400 eggs. It is quite probable that 
this one shipment of lily-of-the-valley, 
after being distributed by sale to 
florists for growing, might have es- 
tablished the nematode in not one but 
many new areas. Its introduction 
into our remaining potato-growing 
areas would be a disaster as it has 
proved to be on Long Island and in 
some parts of £uro]>e. 

The interception of the golden 
nematode on these Convallaria was not 
the first interception of this pest nor 
by any means the last. The cysts are 
found frequently not only in soil with 
plants, but they have also been inter- 
cepted adhering to straw and burlap 
bagging. The menace they present is 
heightened by the longevity of the 
eggs within the cysts. H. rostochiensis 
eggs have been known to hatch after 
8 years. 

Part of the Convallaria shipment was 
treated with hot water. The pips were 
immersed in hot water at 118® F. for 
30 minutes, then removed and cooled 
with water after draining for 5 minutes. 
The treatment must be given with care 



THE INSPECTION OF IMPORTED PLANTS 


as the margin between a complete kill 
of the nematodes and serious injury to 
the plants is very small. The plants 
not treated were forced in postcntry 
quarantine in isolated greenhouses. 
After the flowers were harvested the 
plants were destroyed, and the soil and 
the benches then sterilized with steam. 

Our three examples illustrate the 
varied nature of the inspector’s prob- 
lem. The virus diseases arc especially 
difflcult to detect in dormant plants, 
the condition in which most nursery 
stock is imported. It was largely to 
overcome this difficulty that postentry 
quarantine was devised. 

Postentry quarantine is the re- 
quirement that some genera of plants 
be grown under observation of State 
and Federal inspectors, usually for two 
growing seasons, before they are re- 
leased for sale or distribution. The 
plants subjected to this treatment are 
the ones known to be the hosts of 
some serious plant disease that does 
not occur here or is restricted in dis- 
tribution in the United States. In 
most instances the plants are pro- 
hibited from the countries in which the 
disease is known to occur. The post- 
entry provision then applies only to 
other countries in which it might be 
present but unreported. 

Rose will virus {Marmor flaccum- 
Jacieni) occurs in Italy, Australia, and 
New Zealand. Rose plants or cuttings 
arc prohibited from those countries. 
If a nurseryman wishes to import the 
material from any other foreign coun- 
try he will be required to make a legal 
agreement that he will grow the mate- 
rial at a designated place, accessible to 
the State nursery inspector and the 
Federal plant quarantine inspector 
until it is released. In each of the fol- 
lowing two growing seasons the State 
nursery inspector will inspect the 
plants several times. Sometimes he 
will be accompanied by an inspector 
from the postcntry section of the divi- 
sion of plant quarantines. The plants 
will be examined particularly for any 
evidence of rose wilt virus, but also 


r6i 

for other foreign diseases or insects 
which may have accompanied them. 

Postentry inspectors aim must gather 
from the published records information 
on the foreign plant diseases which we 
are attempting to exclude. Summaries 
of the information are then dis- 
tributed to the State inspection serv- 
ices. In that way the State inspectors 
can know the proper time to inspect 
roses, hops, or other postcntry plants 
in their areas, and something of the 
appearance of the foreign diseases. 

Daphne mosaic virus was found in 
1950 in a lot of 250 Daphne mezereum 
plants from Holland. Then the disease 
was known only in Australia and 
New Zealand. It was the basis for 
the inclusion of daphne in the post- 
entry list. The plants were promptly 
destroyed under the supervision of the 
State nursery inspector. 

At the end of the second growing 
season those postcntry plants which 
have remained free of diseases and 
insects new to the United States arc 
released from quarantine. 

The postcntry inspection makes it 
possible to examine the plants when 
they are in leaf and at times favorable 
for detection of the particular diseases 
for which they arc quarantined. In 
1953 there were 50 genera of plants 
that must be grown in postentry 
quarantine when imported from cer- 
tain parts of the world. Besides them, 
a blanket provision includes fruit 
and nut plants. 

Donald P. Limber has been em- 
ployed as a plant pathologist in the 
Department of Agriculture's plant quar- 
antine work since rg24. He is now plant 
pathologist of the postentry section of the 
Bureau of Entomology arid Plant Quaran- 
tine stationed at Hoboken^ N. J, 

Paul R. Frink, a native of Nebraska^ 
received his graduate training in plant 
pathology at the University of Nebraska, 
He has worked with several Government 
agencies since ig^i and with the division 
of plant quarantines since ig42. He is 
employed as a plant quarantine inspector 
at the San Francisco Inspection Station, 



YEARBOOK OF AGRICULTURE 1953 


162 

Protection 

Through 

Quarantines 

Horace S. Dean 

Modern transportation in the air age 
brings us close in time to lands far 
distant in point of miles. Travel for 
business, recreation, and cultural pur- 
poses stimulates interest in exotic 
plants, fruits, and other plant products. 
Modern commerce and world econ- 
omy are conducive to an international 
flow of plants and plant products — 
together with the plant diseases which 
these materials may carry — across 
international boundaries, including 
our own. 

Were there no barriers to plant dis- 
ease entry, the United Stales could 
readily become the habitat for a host 
of plant diseases which are not now 
known in this country and which, if 
introduced and established here, could 
become the cause of untold additional 
annual losses in field crop, fruit, hor- 
ticultural, floricultural, and forest pro- 
duction. 

We take comfort, however, in the 
provisions of the Plant Quarantine 
Act of 1912, as amended, by which 
many safeguards are erected against 
entry and establishment of foreign 
plant diseases as well as against the 
domestic spread of introduced diseases. 
The safeguards consist of embargo and 
regulatory provisions in both foreign 
and domestic commerce. The follow- 
ing remarks relate only to the pro- 
visions of the Act concerning plant 
diseases. 

Jn order to ,jrcvent the introduction 
of any plant disease that is not known 


in this country or that is not widely 
prevalent or distributed in the United 
States, the Act empowers the Secretary 
of Agriculture to issue a quarantine 
prohibiting the importation of the host 
materials of that disease and prescribes 
certain legal requirements which must 
precede the issuance of that quarantine. 
They include a determination of the 
necessity for the action, and a public 
hearing on the subject. The quarantine 
notice must specify the host materials 
to be excluded and name the countries 
where the disease occurs. Thereafter, 
and until the quarantine is withdrawn, 
the importation of the named host 
materials from the specified countries 
is prohibited regardless of the use for 
which the materials are intended. 

All nursery stock not excluded by 
such embargo action may enter this 
country only when a permit has been 
issued by the Secretary of Agriculture 
for the importation. If the plants arc 
to come from a country with an official 
phytosanitary inspection system, the 
shipment is to be accompanied by a 
certificate of the proper official of the 
country of origin that the nursery stock 
has been thoroughly inspected and is 
believed to be free from injurious plant 
diseases and insect pe^ts. In the case of 
importations from countries without 
ofl^icial systems of inspection, the ship- 
ment shall meet such conditions as the 
Secretary of Agriculture may prescribe. 

Any plants, fruits, vegetables, roots, 
bulbs, seeds, or other plant products 
not defined as nursery stock in the Act 
may be brought under regulated entry 
comparable to that for nursery stock 
by action of the Secretary. To do so he 
must determine that the unrestricted 
importation of any such plant mate- 
rials may result in the entry of injurious 
plant diseases. Other requirements arc 
similar to those necessary to promul- 
gate a prohibitory quarantine. 

An amendment to the Act that was 
approved July 31, 1947, improves the 
protection against foreign plant disease 
entry by making provision for growing 
nursery stock under postentry quaran- 



FROTCCTION THROUGH QUARANTINES 163 

tine by, or under the supervision of, the cign countries are examined for unau- 
Department of Agriculture, for the thorized plant material, 
purpose of determining whether it may The plant material that is permitted 

be infested with plant pests not dis- entry into the United States is in- 
cernible by port-of-entry inspection spected for the presence of plant pests, 
methods. Should there be such infec- The more hazardous of the plant 
tion, the Secretary is authorized to propagating materials that arc per- 
prcscribe remedial measures to pre- mitted entry are routed, upon ar- 
vent the spread. This feature is partic- rival, for intensive inspection at 
ularly useful in the instance of virus specified locations where specialized 
diseases. facilities are available, both for in- 

spection and for whatever treatment 
The various quarantines and is required. These facilities arc main- 
orders issued under authority of the tained at Hoboken, N. J. (in the Port 
Act and now in force cover practically of New York); Miami, Fla.; Laredo, 
the entire field of foreign host mate- Tex.; San Francisco and San Pedro, 
rials — plants and plant parts, including Calif.; Seattle, Wash.; Honolulu, 
fruits, vegetables, seeds, several filjers, T, H.; and San Juan, P. R. Certain 
and cut flowers — likely to carry inju- designated kinds of plants arc then 
rious plant diseases into this country, released only for growing by the im- 
Several of the quarantines and orders porter pending further observation for 
are issued under authority of both the plant pests — particularly virus dis- 
prohibitory and regulatory parts of the eases — not detectible at the time of 
Act. entry. 

A number of the foreign quarantines Not all importations are inspected 
cont^n provisions complementing do- with equal thoroughness because that 
mestic plant disease control programs, would be loo costly and, according to 
For example, a feature of Nursery experience, unnecessary. The Depart- 
Stock, Plant, and Seed Quarantine ment therefore places greater empha- 
No. 37 regulates the entry of barberry sis on the inspection of some classes 
seeds and plants in harmony with the of plants and plant products than on 
provisions of Federal domestic Stem others (according to known or po- 
Rust Quarantine No. 38. Another fea- tential risk of entry of plant diseases) 
ture of Quarantine No. 37 prohibits in order to afford the maximum de- 
the entry of citrus seeds into Florida, gree of protection possible with the 
where there is a comparable State facilities available. The State plant 
quarantine against citrus propagating quarantine agencies support the De- 
materials from other States. partment in connection with the 

The quarantines and orders are en- postentry quarantine work by assum- 
forced by plant quarantine inspectors ing the primary responsibility for the 
at the principal ports of entry. The growing-season surveillance of im- 
inspectors are assisted through cooper- ported plants held under postentry 
ative relations by their associates in the quarantine conditions. Certain States 
Customs, Immigration, Public Health, contribute materially to the entry 
and Postal Services, and the Bureau of control program at ports of arrival. 
Animal Industry and by collaborating The Department itself makes nu- 
Statc plant quarantine services. The mcrous importations in connection with 
total effort of those services and the its program for improving field and 
foreign inspection and certification of vegetable crops, the horticulture, 
plants before shipment provide an cf- floriculture, and forestry of the coun- 
fectivc program for the enforcement of try, and for kindred experimental 
the quarantines and prevention of the and scientific reasons. These intro- 
entry of plant diseases. Carriers and ductions, which may legally include 
cargoes, mail, and baggage from for- material prohibited entry, are sub- 



rEARlOOK OF AORICULTURi 1953 


164 

jected to evcn^orc rigid control and 
S2deguards on entry than are intro- 
ductions by the public. 

As to domestic commerce, the Act 
in some respects is more adaptable 
to the needs of a program for preven- 
tion of the spread of plant diseases 
because it gives the Secretary au- 
thority to embargo or regulate the 
moveruent of “stone or quarry prod- 
ucts, or any other article of any 
character whatsoever, capable of car- 
rying any dangerous plant disease,” 
as well as authority to embargo or 
regulate the movement of nursery stock 
and other plants and plant products. 

In a manner similar to the proce- 
dure prescribed in the field of foreign 
commerce, the Secretary may quar- 
antine any State, Territory, or Dis- 
trict of the United States, or part 
thereof, whenever he determines it is 
necessary to do so in order to prevent 
the spread of a new or not yet widely 
prevalent or distributed dangerous 
plant disease. He is directed, when 
public interest will permit, to provide 
by regulation for the msp>ection, 
treatment, and certification of the 
regulated materials in order to govern 
their movement from the quarantine 
area. 

Unless the Secretary has issued a 
quarantine to prevent the domestic 
spread of a dangerous plant disease, 
the Act sp'^cificily provides that a 
State, Territory, or District may en- 
force a measure for the same purpose. 
The Secretary has discretionary au- 
thority also to cooperate with States 
to carry out their quarantine measures. 

The quarantines and regulations 
governing the domestic movement of 
plant materials may be divided 
roughly into two classes — those gov- 
erning interstate movement on the 
mainland of the United States and 
those governing movement between 
the off-shore territories and possessions 
and the mainland, or other off-shore 
territories and possessions. These do- 
mestic quarantines on the mainland 
are usually associated with disease 
control projects of the Department. 


The States help materially in carrying 
out these projects, including enforce- 
ment of related intrastate quarantines. 
Further assistance in the domestic 
quarantine enforcement is derived 
from the inspection by Federal inspec- 
tors of shipments of quarantined 
materials moving interstate through 
traffic centers. Although the Act pro- 
vides for such protection, practical 
enforcement problems resulted in lift- 
ing the only quarantine of the main- 
land for the protection of an off-shore 
area. Only three quarantines of off- 
shore areas exist w^hich arc designed 
for plant disease reasons to protect the 
continental United States; namely the 
quarantine (No. 16) of Hawaii and 
^erto Rico on account of sugarcane 
diseases, and the quarantines of Hawaii 
on account of citrus canker (Nos. 13 
and 75). 

The Act also makes especial pro- 
vision with respect to the District of 
Columbia in order that it may have a 
plant quarantine inspection service 
similar to that of the States and Ter- 
ritories. The Secretary of Agriculture 
is authorized to make regulations to 
govern the movement of plants and 
plant products into and out of the Dis- 
trict “in order further to control and 
eradicate and to prevent the dissemi- 
nation of dangerous plant diseases.” 
This part of the Act also confers au- 
thority to compel an owner to take 
sanitary action in case of a nuisance 
situation to prevent the dissemination 
of plant disease. 

Horace S. Dean, a graduate oj the 
University of Tennessee^ entered the em^ 
ployment of the Federal Horticultural 
Board in Washington, D, C,, as a plant 
quarantine inspector in 1923, Except for a 
brief period as a plant pathologist in Cen^ 
tral America, he has worked continuously 
in the field of Federal plant quarantine, 
with special emphasis in later years on 
program administration. He is now assist^ 
ant division leader of the division of plant 
quarantines. Bureau of Entomology and 
Plant Quarantine, 




The Nature of 
Resistance 
to Disease 


S. A. Wingard 

We can get a double economic gain 
if we can introduce into our agriculture 
crops that are naturally immune to 
diseases and thereby avoid both the 
loss from disease and the cost of sprays 
and other ways of combatting disease. 

To do that — as W. A. Orton pointed 
out in 1908 — one has to know the 
problems of heredity, the nature of a 
disease, its governing factors, and the 
type of resistance involved in order to 
adopt the most promising lines of ap- 
proach in breeding. 

Disease resistance in plants is not an 
easy subject to understand. It involves 
the intricate relations between the 
plant — the host — that is being at- 
tacked and the fungus or bacterial 
organism — the parasite — that is doing 
the attacking. 

I’he terms “disease resistance” and 
“immunity” can be used lo denote 
different degrees of the same thing. 
Various degrees of disease resistance 
are possible. Immunity means com- 
plete resistance to disease; immune 
means not subject to attack by a path- 
ogenic organism or virus. 

We might consider othei definitions 
here at the start: A pathogen is a 
parasitic organism or virus whose 
activity causes disease in the host. The 
host is a living organism that harbors 
another organism or virus that depends 
on it for existence. Pathogenic, the 
adjective, means having the ability to 
induce disease. A parasite is an organ- 
ism or virus that lives on the tissues of 

165 




riARIOOK OF AOIICUirURf IF5R 


i66 

another living organism. To inoculate 
is to introduce a micro-organism or 
virus or a material containing either 
into an organism, a culture medium, 
soil, or something like them. To infect 
is to invade an organism and to bring 
about infection. A suscept is an organ- 
ism that is affected or can be affected 
by a given disease. Resistance is the 
ability of a plant to withstand, oppose, 
lessen, or overcome the attack of a 
pathogen. Susceptibility is the inability 
of a plant to defend itself against an 
organism or to overcome the effects of 
invasion by a pathogenic organism or 
virus. 

Immunity is absolute. Resistance 
and susceptibility are relative: A plant 
is either immune or not immune to a 
pathogen, but it may be more or less 
susceptible or resistant. A plant may 
be “slightly susceptible,” “moderately 
resistant,” or “extremely susceptible” — 
but not “moderately immune” or 
“highly immune.” 

The ability of a susceptible plant to 
avoid infection because it possesses 
some quality (such as earliness of 
maturity) that prevents successful in- 
oculation is called escape, or klendu- 
sity. Escape must be clearly distin- 
guished from resistance. Tolerance is 
the ability of a plant to endure the 
invasion of a pathogen without show- 
ing many symptoms or much damage. 
A degree of resistance great enough so 
that no serious economic loss results 
(although there might be considerable 
invasion by the pathogen) is termed 
“practical resistance.” Certain varieties 
of wheat, for example, for practical 
purposes are resistant to Icslf rust — 
the rust causes little loss although the 
plants may become heavily rusted as 
they approach maturity. 

We use the term hypersensitiveness 
to denote such a violent reaction of a 
plant to an attack by an obligate para- 
site (a pathogen that depends on living 
tissue for its nutrition) tl^t the invaded 
tissues of the host are quickly killed, so 
that there is no further spread of in- 
fection. In essence, hypersensitiveness 
is extreme susceptibility, but its prac- 


tical effect, as far as crop Joss is con- 
cerned, amounts to extreme resistance. 
This type of reaction is common in 
cases of infection of many plants by 
rust fungi and some of the viruses. 

The definitions help us avoid some 
wrong ideas about the nature of resist- 
ance and help us understand what can 
and cannot done in plant breeding. 
For instance, a variety that escapes a 
particular disease is not necessarily a 
resistant variety. 

William A. Orton long ago pointed 
out the differences between what he 
termed disease-escaping, disease-en- 
during, disease-resisting, and immune 
varieties. 

He said: “Disease endurance some- 
times results from the ability of the 
plant to grow in spite of an attack, 
cither through exceptional vigor or 
through a hardier structure, as in the 
case of certain melons which better 
survive the attacks of leaf-blight be- 
cause the leaves do not dry out as 
quickly as do those of the ordinary 
melons. Drought-resistant plants are 
often disease-enduring. Watermelons 
from semiarid Russia were for this 
reason the last to succumb to the wilt 
disease when planted in our Southern 
States. 

“Finally, we have disease-escaping 
varieties. Such, for example, arc the 
extra early cowpeas which mature 
before the season for wilt and root- 
knot to develop. These varieties which 
escape disease through earliness or 
lateness are often really very suscep- 
tible. The Early Ohio and o^cr early 
potatoes, which commonly mature 
before the appearance of the late 
blight disease, are among the first to 
succumb to this disease if planted so 
late as to be still immature when the 
moist weather of the late summer or 
early fall enables late blight to spread.” 

E. M. Freeman, a professor in the 
University of Minnesota and one of 
the pioneers in this subject, pointed 
out that little difference exists in the 
real resistance powers of oat varieties 
to the common oat rusts, and when a 
grower is told that an oat variety is 



THE NATU9B Of ftESISTANCE TO DISEASE 


resistant because it usually escapes the 
rust through eariiness of ripening, he 
is led into a serious mistake. The 
essential character of true resistance 
lies in a protoplasmic activity and is 
independent of inoculation accidents. 

A variety may escape a disease 
through certain peculiarities of the 
host. To prove that, Dr. Freeman grew 
a variety of barley in different soils, 
which varied from the normal garden 
soil to those containing about 2 per- 
cent of alkaline salts. Plants in the 
different soils, inoculated by sprays in 
the greenhouse, showed different 
amounts of rust. Those in the stronger 
alkaline soils generally showed less 
rust. The latter, however, when at- 
tacked outside the greenhouse, ex- 
hibited large and vigorous growths of 
the rust. That was undoubtedly due to 
the greater development of “bloom” 
on the barley foliage when it was grown 
in the strong alkali; the bloom caused 
the drops of water to roll off and the 
inoculating material to be lost. The 
bluish color of the plants in the alka- 
line soils and the greater tendency of 
the water drops to run off were quite 
pronounced. There seems to be no 
reason for assuming that there was 
any difference in real resistance to 
rust. 

The economic importance of disease- 
enduring varieties ought not to be 
overlooked by pathologists and plant 
breeders. Effective work in selection 
and breeding may be accomplished 
in the production of disease-enduring 
plants. Some of the best varieties 
of the hard spring wheats commonly 
used in the North Central States, for 
example, come under this class in 
regard to stem rust, since they do 
not possess any appreciable amount 
of resistance. They can endure mod- 
erate attacks of rust, but all go down 
in a heavy epidemic of rust. No sharp 
distinction may exist between disease- 
enduring and disease-resistant varie- 
ties, but for practical purposes well- 
marked resistance can be detected 
readily under proper experimental 
conditions. 


167 

Immune varieties have perfect re- 
sistance. Their production will prob- 
ably always remain rare in comparison 
with those possessing only a partial 
resistance or high powers of endurance. 

E. J. Butler of India in 1918 em- 
phasized the importance of distin- 
guishing between disease resistance 
and disease escape, between avoidance 
of disease and endurance of disease, 
between true immunity and resist- 
ance. He gave several instances of 
the way in which plants may avoid 
a disease to which they are not 
truly resistant. They may be grown 
in areas with a climate that the 
parasite cannot stand. (Many of our 
most important cultivated plants have 
a wider range than their parasites.) 
Or the date of sowing may be changed 
to a period when the temperature 
or humidity is unsuitable for germina- 
tion of the spores of the parasite. 
Varieties may be grown that mature 
quickly before the parasite can do 
them much damage. Instances of 
successful endurance of plants to 
attacks of parasites involve mostly 
the vigor of the plants and can be 
modified by different methods of 
cultivation and manuring. 

True resistance to disease is different 
from those instances in that it depends 
on some structural or physiological 
characters of the plant that prevent 
successful invasion of the parasite. 

C. Brick of Germany in 1919 pointed 
out that susceptibility of plants to 
disease is not due to degeneration, old 
age, and such causes, but usually 
is the result of certain differences in 
the stiiictures of the host itself. He 
believed that a higher content of 
acid, sugar, and tannin also has a 
bearing on the resistance of a variety 
to parasites. He said that some plants 
escape disease because their season of 
blossoming or maturing does not 
coincide with the development of the 
parasite. 

Different types of disease resist- 
ance exist in plants. 

To quote Orton again: “The typi- 



,68 YBARBOOK OF AORICUlTUtE 1953 

cal form of disease resistance involves factory demonstrations of cases whw 
a specific reaction on the part of the resistance to highly adapt^ parasites 
host cell against a true parasite, a is due to thickened epidermis, develop- 
character developed in nature in the ment of hairs, etc., are lacking, 
evolution of the species and strength- It has, on the other hand, been 
ened in cultivated plants through the shown . . . that germinating spores 
work of the breeder. of fungi often penetrate the epidermis 

“Less imp)ortant from the breeder’s of plants they can not parasitize, 
standpoint aie plants resistant through and are killed forthwith by the cells 
(a) structural differences, (b) disease they attack. It is hard to under- 
endurance, and (c) disease avoidance, stand why a thick cell wall should 
The evidence indicates that the resist- protect from infection a leaf which 
ance is due to a specific protective has many thousand openings as 
reaction of the host cell against the breathing pores through which a 
parasite. So in plants the evidence fungus might enter. . . . 
leads us to believe that more is in- “Resistance due to structural causes 
volved than the acidity of the cell docs occur in troubles due to wound 
sap or the chemo tactic effect of sugars parasites, a fruit or a tuber with a 
or other food substances. thick rind being thereby less liable 

“The first group is the most im- to bruising; as there may be an 
portant, relating as it docs to diseases indirect connection, a plant of more 
due to the most highly developed open habit of growth being thereby 
parasites, such as the rusts, mildews, less subject to attack by fungi which 
and other injurious fungi. The evi- require moisture for their develop- 
dence indipates that the resistance is ment.” 

due to a specific protective reaction Observations by other early scien- 
of the host cell against the parasite.” tists showed that mechanical immunity 
Orton was not sure that the re- may have some significance in certain 
action between host cell and fungas instances but cannot be accepted as 
parasite in plants w'as the same as a universal phenomenon; natural im- 
that described for some forms of munity does not depend on the ana- 
immunity in man and the higher tomic^ peculiarities of plants, but on 
animals, in wliich substances in the properties of their cytoplasmic cell 
blood serum neutralize the toxin contents and on active resistance of 
excreted by the invading bacteria host plant cells, usually accompanied 
and assist in the de.struction of the by a complicated physiological re- 
latter. He stated, however, that the action in response to penetration by 
evidence in plants leads us to believe the parasite. 

that more is involved than the acidity Other workers just as strongly 
of the cell sap or the chemotactic contend that resistance or immunity 
effect of sugars or other substances, in plants is due to morphological 
He said: “The delicacy of the factors. Still others have confused 
reaction may be better understood the “disease-enduring” and “disease- 
if we recall the fact that it is adjusted escaping” varieties with resistant or 
to repel specific invaders. A plant immune varieties, 
resistant to one disease may be quite 

susceptible to another. General hardi- The causes for apparent resisti- 
ness is also another matter. A plant ance or immunity to a disease and 
may be resistant to cold and yet tolerance to a disease arc rather easily 
extremely susceptible to the attack of explained. But when we consider the 
some parasite . causes for true immunity and resistance, 

“Structural differences do not seem we find ourselves involved in a maze 
to play much part in enabling plants of anatomical, physiological, biochem- 
to resist the true parasite. Satis- ical, and ecological evidence and 



TMi NATUtE OF tESISTANCE TO DISEASE 

theories, which have been offered, 
some to explain specific cases of re- 


sistance and the others to explain the 
problem in general. 

Before entering into a discussion of 
them, we should have in mind the 
fact that what we commonly think of 
as infection consists of two stages. The 
first is the entrance of the parasite 
into the tissues of the plants. The 
second is the establishment of para- 
sitic relationship with the host. Some 
fungi, such as the rusts, enter many 
plants from which they cannot obtain 
nourishment and therefore perish be- 
cause they cannot accomplish the 
second stage of infection. 

One realizes, then, that immunity 
and resistance may be due to the 
plant’s morphological and anatomical 
characters that prevent the first stage 
of infection or entrance of the para- 
site into the tissues; or to biochemical 
properties or anatomical characters 
of the tissues of the host, which prevent 
the second stage of infection or the 
establishment of parasitic relationship 
with the tissues of the plant. 

Cases of immunity or resistance due 
to the anatomical characters of the 
host preventing entrance by the para- 
site are frequent. 

Coffee leaf disease usually starts on 
the under surface of the leaf because 
there are few stomata — pores or open- 
ings — on the upper surface. 

Young beet leaves are practically 
immune to attacks by Cercospora hcticola 
because their stomata are so small as 
to be incapable of opening widely 
enough to allow for the entrance of 
the germ tube of the spores of the 
fungus which can only enter the host 
through the mature stomata. 

Some varieties of plums resist brown 
rot caused by Sclerotinia cinerea because 
the stomata soon become plugged 
with masses of small parenchymatous 
cells. The toughness of the skin, the 
firmness of the flesh, and high fiber 
content also arc characteristics that 
make varieties of plums resistant to 
brown rot. As ripening progresses, the 
texture of the resistant varieties rc- 


169 

mains firm, while that of the suscep- 
tible becomes softer. 

The stomata of Kanred wheat, 
which is resistant to certain strains of 
Puccinia graminis tritici^ arc said to be of 
such a nature as to shut out most of 
the fungi. Wheat has a tendency lo 
keep many of its stomata dosed. 

It may be that a secretion of the 
rust fungus makes the mechanism of 
the stomata inoperative and they re- 
main closed, thus excluding the fun- 
gus. In the Kanred wheat, which has 
smaller stomata, those peculiarities 
might be more effective in excluding 
fungi than in varieties of wheat with 
larger stomata. 

The resistance of Citrus nohilus to 
citrus canker is due to a broad ridge 
over-arching the outer chamber of the 
stomata, which practically excludes 
water from them, thus preventing the 
entrance of the canker bacteria. 

The resistance of some carnations 
to rust may be due to the type of 
stomata that make it impossible for 
the rust hyphac to penetrate the leaves. 

The resistance of some varieties of 
barley to rust is due to the bloom — 
waxy coating — on the leaves, which 
keeps drops of water from adhering 
to them. Consequently the rust spores 
cannot germinate on them. The more 
waxy varieties of. raspberries and 
grapes arc less damaged than others 
by cane blight. 

Mr. B. F. Lutman, uf the Vermont 
Agricultural Experiment Station, after 
investigating the resistance of potato 
tubers to scab {Actinomyces scabies)^ re- 
ported that at the head of the resist- 
ant class stand the potatoes of the 
russet type and that thickness of skin 
determines resistance of tubers to scab. 

The hairs on the leave's may in- 
fluence resistance of plants. Varieties 
of potatoes with small, hairy leaves 
and open habit of growth dry quickly 
after wetting and are less liable to 
infection by late blight than other 
kinds, but perhaps the open habit of 
growth (and not the abundant hairs) 
is the chief factor with such varieties. 

Some technicians believe that hairs 



YEARBOOK OF AGRICULTURE 1953 


170 

seem to save the under surface of the 
apple leaves from infection with the 
apple scab fungus (Venlurta inaequa- 
lis)y while Venturia pyrina readily at- 
tacks the smooth under surface of the 
pear leaves. That belief, however, 
does not agree entirely with other 
observations in regard to the scab on 
apple. Frequently most, if not all, 
primary infection by apple scab is on 
the lower surface of the leaves. It is 
also a matter of common observation 
that the young fruits, while still cov- 
ered with down, are very susceptible 
to scab. 

The very hairy common mullein 
(Verbascum thapsus) is attacked by six 
leaf-inhabit* ng parasitic fungi, but 
only three are reported as occurring 
on the moth mullein (V. blattaria)^ 
w'hich is free of hairs. 

Variations of form or structure in 
certain varieties may influence suscep- 
tibility. As an example: Varieties of 
pears with an open channel from calyx 
to core are most susceptible to Fusarium 
petrejaciens, a fruit-rotting organism. 
The immunity to loose smut in barley 
has been attributed to the closed flow- 
ers found in the resistant varieties. In 
closed flowers the stigmas are not ex- 
posed to infection. The rare occurrence 
of ergot on wheat is said to be due 
to the brief and irregular openings of 
the glumes at maturity. 

Let us next consider cases of im- 
munity due to incompatibility of the 
invading organism with the tissues of 
the plant — the failure of the organism 
to accomplish the second stage of in- 
fection, which is the establishment of 
parasitic relationship with the host. 

The prevention of this second stage 
of infection sometimes is due to the 
anatomical characters of the host tis- 
sue. More often it is due to biochem- 
ical properties of the cells. 

Examples of structural characters 
influencing resistance have been ob- 
served in plums attacked by brown 
rot {Sclerotinia cinera) and potatoes in- 
fected by tut^er rot {Pythium dehary- 
anum). Some varieties of plums be- 


come more susceptible to rot when 
they begin to ripen. That is due to a 
softening of the middle lamellae be- 
tween the cells, which allows the 
fungus to force its way through the 
tissues more rapidly. 

In making a physiological study of 
the parasitism of Pythium debaryanum 
on the potato tuber, Lena Hawkins 
and R. B. Harvey, of the Department 
of Agriculture, found that the fungus 
secretes a toxin that kills the cells and 
an enzyme that breaks down the mid- 
dle lamellae but does not affect the 
secondary thickenings to any extent. 

They observed that White McCor- 
mick, a potato very resistant to Pythium 
debaryanum^ has a higher crude fibc:r 
content than that of susceptible vari- 
eties. The higher fiber content they 
attributed to more secondary thicken- 
ings in the cell walls. Resistance to 
infection is apparently due to the 
resistance of the cell wall to mechani- 
cal puncture by the invading hyphae. 

When we come to consider cases of 
immunity due to biochemical proper- 
ties of the cells, we are confronted with 
many complicated theories and specu- 
lations, all of which support the theory 
that such immunity Ls due to a chemi- 
cal interaction between the host and 
the parasite. 

E. C. Stakman, of the University of 
Minnesota, in 1919 showed that resist- 
ance of some plants to rust is due to the 
dying of the cells surrounding the 
point of infection and the consequent 
starving of the obligate parasite. That 
is called supersensitivity, or hyper- 
sensitiveness. The cause has not been 
determined, but it might be of bio- 
chemical origin. 

J. G. Leach, then of the University 
of Minnesota, stated that in a variety 
of bean highly resistant to anthracnose 
seldom more than one or two cells are 
attacked and that resistance seems to 
be due to the inability of the fungus to 
obtain nourishment from the living 
protoplasm of the bean. 

Tannin, often found in vegetable 
cells, generally is toxic to fungi. 

M. T. Cook and J. J. Taubenhaus, 



THE NATUEE OF lESISTANCE TO DISEASE 


of the Delaware Agricultural Experi- 
mcnt Station, pointed out that fc^ans 
are more susceptible to anthracnose 
(Colletotrichum lindmuthianum) during 
the stages of their growth when the 
enzyme that acts on gallic acid in the 
cells to form tannin is least abundant. 
In apples, pears, and persimmons, and 
other fruits, the enzyme is less abun* 
dant when the fruits are ripe than 
when they are green. That explains 
the lower resistance to rots in the ripe 
fruits. 

Cook found also that tannin affects 
differently the various species of the 
Endothia fungus. It inhibits E, radicalis 
and E. gyrose, E, parasitica^ the cause 
of chestnut blight, is retarded at first 
but later is able to feed on the tannin. 

Acidity of the cell sap evidently 
plays some part in resistance of grapes 
to black rot (Guignardia hidwellii). 
Varieties that contain less tartaric acid 
in their cell sap show greater resistance 
to the rot than those containing greater 
amounts. Botrytis cinereay a parasitic 
fungus, is repelled by certain acids in 
plant cells. The resistance of grapes to 
powdery mildew {Uncinula necator) also 
has been correlated with the acidity of 
the cell sap. The resistant varieties 
contain more tartaric acid than do the 
less resistant kinds. 

O. Comes, an Italian investigator, 
has drawn attention to the fact that 
Rieti, a variety of wheat strongly re- 
sistant to rust in Italy, has a more acid 
sap than that of any of the other less 
resistant kinds he tested. The loss of 
resistance in Rieti when it is grown in 
warmer localities than its native area 
is correlated with reduced acidity of 
the sap. 

M. Popp, a German scientist who 
investigated the hydrogen-ion concen- 
tration and natural immunity of plants, 
found that infections of pathogenic 
bacteria cause the plants to respond 
with variations of the hydrogen-ion 
concentration. Immediately aher in- 
fection the acidity decreases. At the 
end of the incubation period the 
acidity rises. If the plant can with- 
stand the infection, the acidity then 


171 

falls back to normal. If the plant can- 
not withstand the infection, hydrogen- 
ion concentration rises to a high level 
and then falls, usually below normal. 

Other cell contents seem to account 
for some types of resistance. 

Biologic species of parasitic fungi are 
sensitive to minute differences in the 
amounts of albumin in host plants. 
Maybe that explains why some varie- 
ties are resistant to certain biologic 
strains and not to others. 

J. C. Walker, of the University of 
Wisconsin, observed that the smudge 
fungus, Colletotrichum circinanSy normally 
attacks only white onions and not 
those with red or yellow papery scales. 
But if he removed the colored scales, 
the colored onions then became sus- 
ceptible to the smudge disease. Acting 
on the hunch that the pigments were 
associated with the resistance of the 
colored onions, he extracted the pig- 
ments with water and found that in the 
extracts the spores of the smudge 
fungus would not germinate normally. 
Nor would the fungus grow, although 
it developed normally in similar ex- 
tracts of white-scaled onions. 

The extracts of red and yellow pig- 
ments were analyzed later by K. P. 
Link and H. R. Angell, Dr. Walker’s 
associates. They found the toxic sub- 
stances to be protocatcchuic acid and 
catechol. They discovered that the 
substances also protect the colored 
onions from certain other diseases. 

H. N. Kargopolova discovered in 
Russia that the cell sap of wheat varie- 
ties immune or highly resistant to the 
leaf rust fungus Puccinia triticina was 
characterized by a high content of 
phenols similar to protocatechuic acid, 
while that of susceptible varieties was 
low or completely devoid of these 
compounds. 

J. Dufrenoy of France has shown 
that in resistant plants the vacuolar 
sap of cells adjacent to those killed by 
the invading pathogen become rich 
in phen9lic compounds. He suggested 
that such elaboration of toxic materials 
is an important factor in the resistance 
of the plant. 



YEARBOOK OF AORICULTURE 1953 


172 

W. N. Ezekiel and J. F. Fudge, of 
the Texas Agricultural Ex]:)eriment 
Station, explained that the general im- 
munity of monocot plants to root rot 
{Phymatotrichum omnivorum) is due at 
least in part to the presence in the roots 
of these plants of minute quantities of 
acids, ether-soluble substances, and 
possibly organic acids or esters. G. A. 
Greathouse and hLs associates of the 
Texas station have shown that the 
resistance of several species of plants to 
phymatotrichum root rot can be ac- 
counted for on the basis of their alka- 
loid content. 

L. I. Miller, of the Virginia Agri- 
cultural Experiment Station, noted a 
close correlation between the ribo- 
flavin content of the leaves of j^caiiut 
varieties and the resistance of the 
varieties to peanut leaf spot (Cercospora 
arathidicola) . The lower the riboflavin 
content, the greater the susceptibility 
to leaf spot. No apparent relationship 
existed between leaf spot resistance and 
the amount of protein, ether extract, 
ash, crude fiber, and nitrogen-free 
extract of the peanut leaves. 

Chemical disease resistance in plants 
offers the biochemist a fertile field for 
research, and in that field we should 
search for the causes of disea.se resist- 
ance in plants. 

We must not regard the different 
degrees of disease resistance displayed 
by plants as fixed and absolute. En- 
vironmental factors modify them pro- 
foundly. In fact, growers qften believe 
that unfavorable rainfall and tem- 
perature arc the direct and only rea- 
sons for the trouble. Temperature, 
moisture, fertility, and reaction of the 
soil markedly affect disease develop- 
ment. Light and the temperature and 
humidity of the air also may be 
important. 

Other factors, such as the age and 
maturity of the plant, may affect dis- 
ease resistance. Thus the observed de- 
gree of susceptibility or resistance in 
any case is a product of many inter- 
acting factors of which the inherent 
susceptibility of the host is only one. 


The degree of virulence of the patho- 
gen, the age and condition of the plant, 
and the environment with its many 
effects on both host and pathogen, ail 
must be suitable before maximum sus- 
ceptibility can be cxpres.scd. 

K. Starr Chester, of the Oklahoma 
Agricultural Experiment Station, has 
compared infection with the operation 
of a complicated lock, every tooth and 
tumbler of which must be in proj>cr 
alincment before the lock will open; 
as the failure of a single correspondence 
between lock and key will prevent the 
act of unlocking, just so the failure of 
any of the many factors required for 
disease expression may entirely inhibit 
the infection. 

We have seen that plants have 
many types and grades of resistance to 
disease. In certain types the resistance 
is due to a reaction of the cells or tis- 
sues of the host to invasion by the 
parasite. 

It is natural, therefore, for us to ask 
whether there is in plants a type of in- 
duced or acquired immunity, such as 
is so important a factor in animal and 
human pathology. A patient, for in- 
stance, recovers from scarlet fever; his 
blood contains antibodies that destroy 
the pathogen; he has acquired an im- 
munity from .scarlet fever and he will 
not again take the disease. Also, his 
blood scrum may be used to prevent 
or cure scarlet fever in other patients. 
Once a child is vaccinated with the 
virus of cowpox, a mild form of small- 
pox, he suffers a mild attack of cowpox, 
and as a consequence he acquires im- 
munity from the more virulent small- 
pox disease. 

Do plants have the same ability as 
animals and human beings to recover 
from one attack of disease and thereby 
acquire an immunity against subse- 
quent attacks of the same disease? If 
not, why not? Much thought has been 
given to those questions. With the ex- 
ception of some of the virus diseases, 
however, we have little or no clear evi- 
dence of the development of acquired 
immunity in plants. There appears to 



THE NATURE OF RESISTANCE TO DISEASE 


be no good evidence of the existence 
of antibodies elsewhere than in the im- 
mediate vicinity of the site of infection 
by fungus and bacterial parasites. Be- 
cause infection by the organisms is usu- 
ally localized, only local immunization 
is apt to occur. 

The organization of plants is much 
simpler than that of animals in the 
sense that each component part is less 
closely linked with the well-being of ail 
the other parts and is less dependent 
for its functioning on them. There is no 
central nervous system and no blood 
or lymph streams. Every living part 
has or may have within it the capacity 
to regenerate the whole plant. There- 
fore we should not draw analogies be- 
tween what occurs in animal disease 
(where any local disturbances or local- 
ized injury may have rapid repercus- 
sions on other parts) and disease in a 
plant (where an organ, such as a leaf 
or branch, may be lost with little or no 
effect on the rest of the plant). 

The only cases in which it has been 
fully demonstrated that infection can 
lead in plants, as in animals, to im- 
munity from subsequent reinfection oc- 
cur in the most completely systemic 
plant diseases that are known — the 
ones that arc catjscd by viruses. 

The virus diseases of plants arc much 
like the diseases of animals in that the 
virus can spread through the entire 
plant and infect all its cells. In several 
virus diseases a previous inoculation, 
or “vaccination,” will confer immu- 
nity against a later attack, even when 
the second inoculation is with a much 
more virulent strain of the virus. 

Tobacco ring spot is an example. 
Its virus produces many dead lesions 
on the leaves of young tobacco plants. 
If the diseased plants arc protected 
and allowed to continue growth, how- 
ever, the severe phase of the disease 
passes, and the new leaves show less and 
less evidence of disease. Finally leaves 
arc produced that arc entirely normal 
in appearance. If the leaves arc inocu- 
lated with the ring spot virus, no 
disease will develop; the plant has 
thus acquired immunity from ring 


*73 

spot. Cuttings may be taken from 
the recovered parts of suck plants 
and from the cuttings new plants 
may be produced. The plants appear 
entirely normal and they arc immune, 
as they cannot again be made to show 
symptoms of ring spot by inoculation 
w'ith the virus. 

These “recovered” plants, however, 
still contain the virus. If juice from 
one of them is inoculated into a 
normal plant, the latter will develop 
typical ring spot symptoms. This type 
of immunity, as in animal medicine, 
is specific. The recovered ring spot 
plant is immune to ring spot but 
not to other virus diseases, just 
as the person who has recovered from 
smallpox or has been vaccinated 
against it is immune to that disease 
but not to any other disease. 

S. A. WiNGARD is head oj the depart- 
ment of plant pathology and physiology oj 
the Virginia Polytechnic Institute and 
plant pathologist oj the Virginia Agricul- 
tural Experiment Station and Extension 
Service^ Blacksburg^ Va. He has devoted 
most oj his time to research on the diseases 
oj tobacco, beans, and tree jruits and jor 
many years has been especially interested 
in the nature oj disease lesistarice in plants. 
Dr. Wingard holds degrees jrom the Ala- 
bama Polytechnic Institute and Columbia 
University, 



Ergot sclerotia producing sexual spore 
stage. 



174 


YEARBOOK OF AGRICULTURE 1953 


Breeding for 
Resistance to 
Disease 


George H. Coons 

In the botanical garden of the Na- 
tional School of Agriculture at Mont- 
pellier, France, stands a statue dedi- 
cated to Professor G. Foex, commem- 
orating his success in saving the grape 
industry of France by resistant varie- 
ties from America. It symbolizes the 
spirit and aim behind man's efibrt to 
breed healthier plants. 

The figure of the weak, old man in 
the statue repre.scnts the grape industry 
of southern France, about to die. Sup- 
porting and sustaining the man is a 
sturdy, young woman, America. 

The statue comrrcmoraics the mod- 
ern application of disca.se resistance as a 
control measure that began about 1870. 
The story of it has great meaning for us 
today. 

Attempts to control powdery mildew 
of grape m Europe by introducing 
rooted American vines for breeding 
purposes had backfired because the 
American grapes had carried with 
them the grape root aphis, or phyllox- 
era. The insect, native of United Stales, 
did no great damage to our wild and 
cultivated grapes because it and its 
host had reached an equilibrium over 
the centuries. Introduced into Europe 
some time before i860, the phylloxera 
found the vinifera types of grape highly 
susceptible. Soon the insect spread 
throughout the entire grape-growing 
area. 

When the vines were first found dy- 
ing under attacks of the root aphis, the 
American varieties were remarkable in 


escaping injury. Attempts were then 
made to substitute the best American 
varieties to replace the dead vines. 
That was not, satisfactory. Next, the old 
French vinifera varieties were grafted 
to the American varieties that had 
maintained vigorous growth despite 
the phylloxera. That procedure was 
more successful. It saved the grape in- 
dustry until hybrids between the re- 
sistant or nearly immune American 
sorts and the French varieties could be- 
bred to furnish better adapted, resist- 
ant stocks on which to graft the vinifera 
scions. 

Powdeiy mildew, phylloxera, and 
dow ny mildew, introductions from the 
United States, revolutionized methods 
of grape culture in Europe. Some of 
the resistant stocks that were developed 
to meet the crisis still are being used 
W'herever vinifera types are grown. 

All that the statue at Montpellier 
tells. And more: It underscores the in- 
ternational aspects of the research pro- 
gram that seeks to control plant dis- 
eases by developing disease-resistant 
varieties. 

That program has many facets. 

Men the world over have searched 
wnthin all species of crop plants sub- 
ject to serious diseases and their cousins 
for breeding materials that carry the 
hereditary factors for resistance. 

They have made worldwide collec- 
tions of germ plasm and comprehen- 
sive assemblages of varieties and strains 
of crop plants and their related species. 

The crop specialists have sought the 
wild forebears of the cultivated sorts at 
the places where the species presum- 
ably originated. They have combed 
the countries where the particular dis- 
eases are endemic in the searcii for 
strains and individuals which, through 
operation of natural selection, might 
have come to bear factors for disease 
resistance. 

For experiments and tests of resist- 
ance, they have chosen sites where in- 
cidence of disease may be expected to 
be extremely high, the exposure severe 
and uniform, and other conditions con- 
ducive to efficient research. 



• KEEDING FOI RESISTANCE TO DISEASE 


All in all, then, breeding healthier 
plants is a long-time program whose 
many phases require diversified yet 
balanced, strong, consistent, and thor- 
oughly coordinated efforts. 

The research has to be international 
and national. It must also be regional 
in scope: Seasons have to be tele- 
scoped; tests of breeders’ strains and 
selections under a range of disease ex- 
posures have to be conducted in many 
areas at the same. time. Delays in pro- 
ducing seed and multiplying clones of 
desirable stocks must be avoided. In- 
creases of seed for commercial pur- 
poses have to go forward promptly and 
efficiently. 

Breeding for disease resistance has 
constituted for many years a major 
part of the Department of Agricul- 
ture’s plant pathological program, in 
which the Bureau of Plant Industry, 
Soils, and Agricultural Engineering 
works in cooperation with Stale agri- 
cultural experiment stations, the seed 
growers, and farmers. 

Outstanding results have come from 
the research. Among them are funda- 
mental biological discoveries that ap- 
ply not alone to plants and their reac- 
tions but shed light and afibrd new 
techniques for solution of probleni.s of 
human and animal dLseasc. They have 
added to our knowledge, especially to 
the concepts concerning man and his 
relations to hi.s environment. They 
have had immediate practical signifi- 
cance. The disease-resistant varieties 
that have been developed and intro- 
duced have added greatly to our own 
wealth and to the wealth of nations. 

An attempt was made in 1 936 to ap- 
praise the contribution of disease-re- 
sistant varieties to American agricul- 
ture. The crop statistics and the dollar 
values of the 8-year average, 1928- 
*935> yycrc compiled then of 

course do not apply to today’s farm 
production and values. But at that 
time the contribution to farm value for 
17 leading farm and vegetable crops 
was placed at 10 percent. 

In the years since 1935, the degree 
of disease resistance in the new vari- 


175 

ctics has been greatly increased, and 
many other disease problems have 
been put in line for solution by the 
introduction of superior types. The 
acreage of many crops has increased, 
and on the expanded acreage the dis- 
ease-resistant varieties have gained 
wider and wider acceptance among 
growers. Almost revolutionary changes 
have occurred as old varieties have 
been replaced bv new productions 
whose chief superiority derives from 
disease resistance. The acreage occu- 
pied by varieties for which it is proper 
to apply the term “disease-resistant” 
has moved from approximately 25 
percent in 1933 to more than 50 per- 
cent. No small part of the increase 
derives from the almost total swing in 
many States to varieties of hybrid 
corn and to the popularity of newer 
kinds of wheat, oats, and potatoes. 
Very generally in the areas of greatest 
production the higher yielding, belter 
adapted, and more disease-resistant 
sorts have replaced the old varieties. 

If we want to j^ut the benefit from 
disease resistance in terms of dollars, 
we may use the early, conservative esti- 
mate of 10 percent, which allows for 
seasons with lessened disease outbreak. 
But we must take into account the 
greater utilization of the superior sorts. 
Wc assume that about one-half of the 
acreage is concerned. Our total farm 
value of crops ranges from about 12 to 
15 billion dollars annually. So we get 
ri figure of Goo to 750 million dollars 
as the annual benefit that comes from 
the use of resistant varieties. 

But the returns must not be based 
alone on the farm value of the crop.s, 
because that omits the marketing 
pliase. In a fanning area, if the farmers 
lose their crop or have a reduced pro- 
duction, those engaged in tlic proces- 
sing, handling, sale, and transporta- 
tion have their means of livelihood 
curtailed. With most crops, wc are 
not far wrong in placing the contribu- 
tion to the processing, packaging, mar- 
keting, and transportation industries 
as equivalent to that of the farm 
value — so that 1.2 to 1.3 billion dol- 



YBAtBOOK OB AOBICUITUBB 1953 


176 

lars is the annual contribution to agri- 
culture of disease-resistant varieties. 

One more aspect: The utilization of 
■disease resistance is not merely the 
stopping of leaks here and there in an 
otherwise profitable agriculture. The 
disease situation with our crops very 
frequently is serious and, almost with- 
out exception, control measures add to 
farming costs. The farmer, as he pays 
for fungicides and their application 
and as he employs other disease-con- 
trol measures — such as sterilization of 
seed and soil — pays a heavy impost to 
the plant pathogens. Control of disease 
by use of a disease-resistant variety has 
been described as the “painless meth- 
od’* that does not levy on the farmer’s 
pocketbook except as he has to pay 
for the care and harvest of a larger 
crop. As increasing crop production 
costs tend to make more and more 
crops marginal, the lessened expense 
for disease control may mean the dif- 
ference between profit and loss from 
the farming business. 

With some crops, notably the sugar 
plants, cereals, and potatoes, disease- 
resistant varieties may spell the dif- 
ference between success and failure. 
Without the kinds resistant to curly 
top, western United States would have 
abandoned the culture of sugar beets, 
and this keystone crop would have 
been lost to irrigation agriculture. 
Without resistance to mosaic and root 
rot, sugarcane culture would have dis- 
appeared in the Sugar Belt of the 
South. The citrus industry must de- 
pend on trees reworked on resistant 
stocks if tristeza should develop in 
the United States at all comparably to 
its advance in Argentina, Brazil, and 
other countries. Wheat production 
has been threatened by a new race of 
the black stem rust, 15B, and new 
resistant varieties are imperatively 
needed. 

The contribution of a disease-resist- 
ant variety includes other things. 
Improvement in yield and quality 
usually has accompanied the resistant 
“variety as a result of better and more 
nearly normal growth or as intrinsic 


improvement, irrespective of the fac- 
tors of resistance. The farmer gives 
better culture to a crop that shows 
promise. Certainly the varieties that 
have gained ready acceptance and 
have moved into position of standards 
in our agriculture — such as Washing- 
ton asparagus; Wisconsin yellows-re- 
sistant cabbage; the numerous wheat 
varieties; Bond, Victoria, and other 
disease-resistant oats; Michigan golden 
celery; the Robust bean; mosaic and 
root rot resistant G. P. sugarcane; 
U. S. 22/3 sugar beet, which is resist- 
ant to curly top; U. S. 215 x 216/3 
and U. S. 216 x 226, the varieties of 
sugar beet that are resistant to leaf 
spot; Katahdin and Chippewa pota- 
toes; and mildew- resistant cantaloups 
(to cite only a few) — all have had im- 
proved commercial quality and high 
capacities for yield, along with disease 
resistance. With the resistant varieties, 
even if the crop is somewhat reduced 
under epidemic conditions, there is 
certainly something to harvest which 
offsets labor costs of production — a 
striking contrast to crops of some of 
the old varieties that were not worth 
harvesting at all. Those intangible 
contributions — stability in rotation sys- 
tems, permanence in the agricultural 
program of an area, and the increased 
security that comes from lessening of 
hazards in crop production — permit 
the farmer to plan with greater 
confidence. 

Almost all of our cultivated plants 
trace back to primitive man. During 
the ages, as people wrested the plants 
from nature and conserved them, they 
must have improved them whenever 
disease outbreak retarded or eliminated 
the less resistant ones. 

The early experiences in which re- 
sistant host plants were found in re- 
gions where a given disease or insect 
pest is endemic have shaped our think- 
ing and forced recognition of the first 
and fundamental principle in breeding 
for disease resistance, namely, that 
where host and parasite are long asso- 
ciated, then in the evolutionary proc- 



tN0 rOi mSISTANCi TO DISEASf 


ess resistant forms are developed by 
natural selection. 

Conversely, when an introduced par- 
asite enters a new environment and 
finds new host plants, conditions may 
be conducive to its growth and spread; 
above all, the pathogen may find a host 
*blant in which no resistance has ever 
been developed. Such plants are at- 
tacked with great virulence. Many of 
our serious outbreaks of plant disease 
trace to the introduction of parasites 
to which our nonresistant crop plants 
immediately succumb. Faced with such 
emergencies, it is almost axiomatic 
to apply this basic concept about host 
and parasite relations that came to 
light 75 years ago when grape culture 
in Europe was threatened with, ex- 
tinction. 

William A. Orton inaugurated in 
the Department of Agriculture breed- 
ing for disease resistance as an effec- 
tive and practical means of disease 
control. His investigations met the spe- 
cific disease-control problems and gave 
the guiding principles to this branch 
of plant pathology. When Erwin F. 
Smith, famed plant pathologist and 
bacteriologist of the Department, had 
completed his studies on the pathology 
of the fusarium wilts of cotton, water- 
melon, and cowpea, he assigned to 
young Orton — fresh from the Uni- 
versity of Vermont — the job of devel- 
oping control measures. Each disease 
problem was solved by the application 
of disease resistance, but each crop 
required a different approach. To 
combat the wilt of cowpea, Orton 
utilized the natural resistance of an 
existing variety after his comparative 
tests on infested soil had shown the 
“Iron” cowpea not only resistant to 
wilt but nearly immune to the root 
knot nematode. 

Against cotton wilt, Orton employed 
methods that are operative today in 
all attempts to improve cotton varie- 
ties — the selection within desirable 
strains of individuals that survive 
under conditions of drastic exposure 
and the proving of the selections by 


177 

subsequent tests of the progeny. Co- 
operating closely with growers and a 
practical cotton breeder — E. L. Rivers, 
who had started some selections in 
1895 at Centerville, S. C. — Orton 
centered his attention on fields of 
highest infestation with the fusarium 
wilt fungus. He subjected to further 
tests the progenies from the individual 
plants that were selected, because he 
soon learned that mass selection alone 
was not effective. In less than 10 
years he produced many highly resist- 
ant varieties — Rivers, Centerville, Dil- 
lon, and Dixie varieties, each, in its 
day, a successful introduction that 
grew well where ordinary types failed 
and each a contributor of germ plasm 
for the use of cotton breeders to im- 
prove their varieties further. 

In developing wilt-resistant water- 
melons, Orton had to go beyond just 
selection and progeny testing. Failing 
to find resistance in edible varieties of 
watermelon, he turned to the highly 
resistant citron melon, used only for' 
feeding livestock, and incorporated 
genes for resistance from it into the 
“Eden” watermelon. Thus he synthe- 
sized a disease-resistant variety by hy- 
bridization. Those investigations ante- 
dated the rediscovery of Mendel’s law, 
whose disciplines would have been ex- 
tremely useful to the young scientist as 
he selected for desirable characteristics 
from the segregates in the F2 genera- 
tion. By further selection, Orton ob- 
tained the wilt-resistant watermelon, 
Conqueror, capable of giving good 
crops despite disease. In repeated tests 
in later years, Conqueror retained its 
qualities, and the problem apparently 
was solved, except, as Orton quaintly 
said, “styles in watermelons changed.” 
Market demand for long melons of the 
Tom Watson type made 4 he round type 
of melon unwanted. But the scientific 
contribution was there, and the genes 
from Conqueror still are used in re- 
search. 

H. L. Bolley, another pioneer in 
plant pathological research, shares 
with Orton the distinction of bringing 
to the fore the possibility of meeting 



YEARiOOK OF AGtICULTURE 1953 


178 

serious disease problems by resistance 
breeding. BolJey discovered in 1900 
that flax wiJt, most serious of all flax 
diseases, was caused by a soil-infesting 
fungus, Fusarium Uni. Bolley added a 
new concept to plant pathology, 
namely, that of flax-sick soil — that is, 
soil infested with Fusarium Uni. Bolley 
extended this concept of soil infesta- 
tion to apply to other crops. He 
pointed out that greatly reduced 
yields below those of virgin soils and 
the so-called “running out of soil” 
might have a biological explanation. 
He used experimental plots, notably 
famed Plot 30, that were highly in- 
fested with Fusarium Uni to develop 
resistant flax varieties: North Dakota 
Resistant 52, North Dakota Resistant 
1 1 4, and Buda, and later, with O. A. 
Heggeness, produced the variety Bison, 
which is still in wide commercial use. 

The investigations of Lewis R. Jones 
and his students, J. C. Gilman, J. C. 
Walker, and W. B. Tisdale, at the 
Wisconsin Agricultural Experiment 
Station, in breeding yellows-resistant 
cabbage firmly established disease- 
resistance breeding as a control meas- 
ure. This wilt disease, caused by Fm- 
sarium conglutinanSy had practically ru- 
ined cabbage production in the rich 
bottom lands near Racine, Wis. 

Building on the work of Orton and 
Bolley, the Wisconsin investigators se- 
lected in 1910 some individual plants 
from fields sustaining an almost com- 
plete loss. Only a few of the remaining 
plants produced heads. The plants to 
serve as seed bearers were critically 
selected from them and brought to 
seed. Then the individual progenies 
were tested on highly infested soil. By 
this technique, yellows-resistant strains 
of Hollander type were obtained. Lat- 
er, Walker and his associates produced 
yellows-resistant strains of other stand- 
ard cabbage varieties. The results with 
the resistant cabbages were dramatic. 
The new strains gave almost a full 
crop where the commercial type failed. 
Before the work was concluded, resist- 
ant types, equivalent in quality and 
productiveness to any nonresistant 


types previously grown, were made 
available to growers. This important 
vegetable crop was saved not alone for 
Wisconsin but for many other States 
where the disease had been introduced. 

The w'ork in Wisconsin stressed the 
influence, of environment upon the 
parasitism of Fusarium. Soil temper- 
ature particularly was found to be 
highly significant in determining the 
ability of the fungus to attack and of 
the host to withstand the parasite. 
Here we have the beginning of the 
concept that di.sease resistance needs 
to be defined not alone in terms of the 
organism-host relation, but as well in 
terms that include the environmental 
conditions as they influence both host 
and parasite. 

The work of W. H. Tisdale at the 
Wisconsin Agricultural Experiment 
Station with flax wilt developed that 
idea further. He showed why some 
varieties of flax resisted Fusarium lim 
only under certain conditions. Tem- 
perature relations determined the in- 
fecting powers of the fungus. Wilt- 
susccptible flax grown with soil 
temperatures below 60*^ F. escaped 
disea.se; grown at 68° F., it was 
completely susce7)tiblc. 

By his experiments, conducted on a 
field that had grown flax continuously 
for a decade, H. D. Barker at the 
Minnesota Agricultural Experiment 
Station clarified a confused situation 
with respect to disease resistance by 
showing that resistance depends on 
the genotype and is not something 
acquired by mere as.sociation of host 
and parasite. The high incidence of 
infection in an experimental field at 
St. Paul, Minn., augmented by inoc- 
ulation of the soil with pure cultures 
of the flax wilt Fusarium., allowed him 
to obtain clear-cut reactions with the 
resistant varieties then available. He 
showed that lines may be selected that 
breed true for resistance and that some 
lines are heterozygous. In his work we 
find the first research that indicated 
that the flax fungus itself breaks up 
into strains. 

With the work on flax, cabbage, 



iREEDING FOR RESISTANCE TO DISEASE 


tomato, and other plants, the concept 
of strains within the pathogens anal- 
ogous to what was known for the 
rusts became established. The success 
in breeding strains resistant to the 
soil-infesting vascular parasites of the 
genera Fusarium and Verticillium is 
impressive. Resistant varieties have 
been obtained for numerous plants, 
including aster, banana, cabbage, 
carnation, celery, chrysanthemum, 
cotton, flax, muskmelon, pea, spinach, 
sweetpotato, tobacco, tomato, and 
watermelon. 

A basic thing in Orton’s work with 
the wilt diseases was that he centered 
attention on fields with the highest 
disease incidence. With the water- 
melon wilt, Orton insisted on pro- 
ducing practically too percent infec- 
tion in the test field. That he did by 
locating the tests in fields whose soil 
was known to harbor the fungus and 
by placing manure infested with the 
causal organism in the melon hills. 
He thereby subjected the plants to the 
severest of both natural and artificial 
exposures. In breeding techniques, 
disease exposures are of paramount 
importance. 

Although in the breeding of rust- 
resistant varieties of cereal, the sig- 
nificance of biological races that exist 
within the rust species was given 
prominence frGm almost the beginning 
of the research, that concept, as we 
have seen, was rather slow in being 
recognized in the disea.se-rcsistance 
breeding programs with vegetables 
and certain other crops. The occa- 
sional failure of resistant types to meet 
disease situations eventually forced 
attention to the problem. 

From the investigations of the cereal 
rusts and smuts, we have the most 
complete delineation of the problem 
to be faced in breeding for disease 
resistance — namely, the play of forces 
within the pathogen and the host, 
both Hying entitles with inherent 
capacities for variation. 

Writing in 1924, William B. Brierley, 
a British scientist, tgok note of the 


179 

growing evidence of existence of bio- 
types within common species of para- 
sitic fungi and pointed out that the 
true relation is between “host-strain” 
and “parasite-strain.” That biotypes 
exist within our species of pathogens 
is now commonly accepted. Compari- 
son of isolates on the basis of physio- 
logical reactions reveals the species to 
be composed of many subgroups that 
fall within the limits of the species, 
but differ among themselves; notably, 
this also is the situation wdth respect to 
pathogenicity. By means of the host 
plant and its varieties and segregates, 
it has been possible to demonstrate 
existence of a wnde range of biotypes. 
The complexity of the disease pro- 
ducers, therefore, parallels that of the 
crop plant. We are thus concerned 
wdth the genetic make-up of the host 
and the parasite. 

In seeking to explain the rather 
broad adaptation of resistant varieties 
as they were developed, some of them 
many decades ago, and their continued 
value in agriculture, we are led to 
another principle in disease-resistance 
breeding. 

The first succc.sscs were obtained 
with a group of soil-infesting species. 
All had the characteristics of produc- 
ing such severe disease exposures that 
only those plants survived that were 
truly resistant. Progeny tests conducted 
on the same soil eliminated the “es- 
capes” and concentrated attention on 
truly resistant portions of the popula- 
tion. But in addition to those charac- 
teristics of the early experiments, the 
research w^as conducted in the field 
and against the full range of organisms 
that would be likely to accumulate in 
years of intensive agriculture. Thus 
we may suppose that a fairly large 
array of biotypes w^as included in the 
pathogenic material. A relatively high 
degree of resistance was sought and, 
as such, it would be likely to manifest 
itself similarly against most biotypes of 
the organism. Too often in subsequent 
research, similar breadth of exposures 
has net been sought, nor have criteria 
for resistance been high enough. 



YEARBOOK OF AORICUITURE 1953 


i8o 

From this background and with 
such diverse plants as grape, cotton, 
watermelon, flax, tomato, and the 
cereals, a system of breeding has 
evolved. We have learned that wher- 
ever a serious disease has occurred in 
nature, plants carrying factors for 
resistance may be found, because with- 
out such qualities there would have 
been no survival. 

From the research with Fusarium^ 
we are taught to concentrate on fields 
having nearly total and uniform ex- 
posure to a given disease complex. 
The successful outcome may largely 
be attributed to the cliaractcristics of 
the Fusaria to produce almost cem- 
plete infestation. Thus the parasite 
cooperated by pointing out the plants 
to select and safeguarded against mis- 
take. Should factors for resistance be 
lacking in the cultivated varieties, then 
we have learned to seek jhe genes in 
related resistant species and to incor- 
porate them into the genotype. Al- 
ready a body of knowledge has been 
built up concerning the inheritance of 
the factors for resistance. 

Some form of inoculation procedure 
has commonly been found necessary as 
insurance that the work will not re- 
ceive discouragement and setback by 
selection of individual plants that arc 
mere ‘‘di.seasc-cscapcs” — a bugbear in 
all such breeding. Here the plant pa- 
thologist, by learning how to create a 
localized epidemic in the experimental 
field, may make a highly significant 
contribution. Other logical steps along 
the course set by the ])ionecr research 
are the location of test fields at jilaces 
where environmental factors are con- 
ducive to epidemic outbreak, the 
manipulation of cultural operations to 
increase disease exposure, the main- 
tenance over the years of plots tc insure 
heavy soil infestation, and other similar 
measures to insure maximum exposure 
to diseases. 

We have learned the significance of 
the existence within both host and 
parasite of species of biotypes that 
differ wdth respect to their physiologi- 
cal reactions — using this term in its 


broadest sense. I cannot overstress the 
importance of this. 

If the breeding is to succeed, if it is 
to have wide applicability and meet 
more than some restricted situation, 
and if it is to avoid disappointments, 
then there must be a utilization of the 
widest po.ssible array of genotypes of 
both host and pathogen. The former 
could provide a broad base for selec- 
tion and could provide genes for high 
resistance, and the latter could give as 
broad a base of exposure as possible. 

How a crop plant may be tailored to 
fit a disea.se complex created by the 
niultipHcity of biotypes within a path- 
ogen is shown by research on wheat 
and oat diseases, as di.scus.secl in an- 
other chapter, page 33. Even though 
the parasites are on the move, and a 
once-prized variety may pass out of 
the picture because cf a disea.se, a de- 
featist altitude is not warranted. 
Nevertheless, the fiicts are patent that 
unless the exposure encompasses about 
the ordinary range of virulence ex- 
hibited by the gamut of biolypes of the 
pathogen likely to be encountered, 
the resistant type as developed may be 
very restricted as to its area of adapta- 
tion and short-lived as an agricultural 
variety. 

Thk success of these classic research- 
es lead to the application of the prin- 
ciples and techniques with those dis- 
eases wdth which it was possible to 
produce high incidence of infection. 
The successful results of the investiga- 
tions that stemmed from the early re- 
search constitute highlights of accom- 
plishment. Subsequent sections of this 
Yearbook give, by crops, the results in 
detail. There has been extensive appli- 
cation of the techniques of disease- 
resistance breeding. My familiarity 
with the sugar-plant investigations 
leads me to draw upon them to show 
how it has been possible to broaden 
the scope of this method of disease 
control. These extensions fall, into three 
groups: Breeding for resistance to virus 
infection, breeding for resistance to 
necrotic diseases, and the combinitig 



BREEDING FOR RESISTANCE TO DISEASE 


in one variety of resistance against 
several disea^s. 

Control of virus diseases by breed- 
ing resistant varieties is well exempli- 
fied by sugarcane mosaic. When that 
disease was first definitely identified as 
the cause of decline of sugarcane pro- 
duction in the subtropical cane-grow- 
ing districts of the Western Hemisphere 
probably no recent plant disease 
aroused more public attention. The 
early work of identification and clari- 
fication of the etiology among a welter 
of speculations and the discovery by 
E. W. Brandes at the Department of 
Agriculture that a plant-louse, Aphis 
maidis^ is the vector are dealt with in 
another section. These discoveries 
came shortly after H. A. Allard’s fun- 
damental studies on tobacco mosaic 
had shown that insects can transmit 
virus infections. The first control of 
sugarcane mosaic was by introduction 
of mosaic-tolerant P. O. J. varieties 
from Java, where the disease had long 
been endemic. The P. O. J. varieties 
were introduced into Louisiana in 
1 926, when the sugarcane industry was 
about to fail. They stemmed the tide 
of bankruptcy and saved the industry. 

Then began extensive breeding for 
the control of sugarcane disease in 
which lessons learned from experience 
in Java, where mosaic was endemic, 
were applied to the American prob- 
lems. The research centered on hy- 
bridizing Java and Indian sugarcanes 
that derived their resistance from wild 
sugarcane, Saccharum spontancuni, with 
Saccharum qfficinarum, the noble cane. 
It was necessary to find a place in the 
United States where the sugarcane va- 
rieties would flower in order to make 
the hybrids and backcrosses. Canal 
Point, Fla., on Lake Okeechobee, was 
chosen for the experimental work. 
Then countless seeds had to be grown 
and progenies selected in the fields on 
the basis of agronomic characteristics. 
After this screening to obtain eco- 
nornic types, the seedlings that re- 
mained were subjected to a further 
winnowing process under severe ex- 


181 

posure to sugarcane mosaic. Of the 
thousands of seedlings started in the 
first 10 years of work, only three — 
C. P. 807, C. P. 28/11, and G. P. 28/. 
ig__vvere introduced as economic 
varieties. These three and two impor- 
tations — Co. 281 and Co. 290 — be- 
came standard canes and were great 
advances over old varieties formerly 
cultivated. 

The early breeding work at Canal 
Point was dominated by the need to 
find immediate replacements for D 74 
and Louisiana Purple, old varieties 
that had failed because they were sus- 
ceptible to mosaic, root rot, and red 
rot. The research was somewhat hit- 
and-miss. Beginning in 1928, a pro- 
gram of purposeful crossing, backcross- 
ing, and selfing, accompanied by se- 
lection, was instituted. The plan sought 
to combine the favorable factors from 
tw'o or more varieties of proved value 
into a single variety. The most impor- 
tant hybridizations have been those 
that sought to nobilize the interspecies 
crosses between .S’, qfficinarum and S, 
spontaneumy as well as S. bar her i. These 
crosses have given rise to disease-re- 
sistant varieties that increased the unit 
yield several fold and are adapted to 
many soils and climatic conditions. 

The success of the breeding program, 
which in essence has been the progres- 
sive nobilization of the resistant wild 
plant by application of genetic tech- 
niques while retaining its character- 
istics for disea.se resistance, is evidenced 
by the strccim of disease-resistant, high- 
ly productive, and high-quality canes 
that have been introduced. Bearing in 
mind that the designation “G. P.” de- 
notes a selected sugarcane coming 
from the Canal Point breeding station, 
and that no cane could succeed under 
Louisiana conditions if it were not re- 
sistant to mosaic and other diseases, 
the census of varieties in u.se in Louisi- 
ana in 1951 showed that about 90 per- 
cent of the acreage was planted with 
disease-resistant G, P. varieties. 

Equally dramatic in portraying 
the control of a virus disease is the 



YEARBOOK OF AGRICULTURE 1953 


182 

breeding for the control of sugar beet 
curly top. With the sugar beet, it was 
not necessary to go to wild species of 
Bela for the genes for resistance h>ecause 
the commercial sugar beet as produced 
by European breeding was open-pol- 
linated and extremely heterozygous. 
In the fields where curly top appeared 
early, every plant w^as affected. All but 
a very few resistant ones were rendered 
worthless, Ijeing curled and stunted. 
In those fields the complete and uni- 
form exposure functioned to guide se- 
lections. Eubanks Carsner, Dean A. 
Pack, and others selected resistant in- 
dividuals and by simple mass selection 
produced the first curly top resistant 
variety, U S. i. The variety was only 
moderately resistant; only about one- 
fourth of the plants in the population 
were resistant and the others were sus- 
ceptible. In comparison with almost 
complete failure of commercial strains, 
however, it was outstanding. It offered 
hope to an industry about to quit and 
may be said to have held the line until 
more resistant strains could l>e bred by 
continued mass selection. The curly 
top resistant varieties scored a spec- 
tacular success and saved the sugar beet 
crop for western United States. It pre- 
sents a second example of control of a 
virus disease by breeding for resistance 
to disease. 

The early successes in obtaining 
resistant plants were confined mostly 
to the diseases in w'hich the reaction 
between the host and pathogen was, 
as Orton phrased it, “very delicate.” 
Such a highly specialized relationship 
would exist with diseases produced by 
the most highly developed parasites or 
with pathogens whose activities were 
intracellular. The rust fungi, together 
with the powdery mildews, and possi- 
bly the viruses, usually are considered 
as exhibiting the highest forms of para- 
sitism. With them, therefore, we could 
expect that breeding investigations 
W'hose objective is to upset the relation 
of pathogen strain to host strain would 
i>e likely to be fruitful. The success in 
breeding varieties resistant to fusarium 


wilt w^as with organisms belonging in 
the subsection of this genus in which 
the parasite invades the water-con- 
ducting vessels of the stem and induces 
systemic poisoning — a specialized form 
of parasitism. No cases have been 
reported in which resistance has been 
obtained against the Fusaria that cause 
necrosis of roots, stems, or leaves. 

It is therefore of interest to consider 
diseases of the necrotic type to find if 
they offer possibilities for control by 
breeding. As I indicated, it has been 
more or less accepted that the general 
run of organisms such as Sclerotinia 
libertianay Sclerotium rol/siiy Pkymatotri* 
chum omnivoTumy Fusarium solani and its 
allies, Botrytis fipecieSyPenicillium species, 
RhizopuSy and others occur widely in 
nature as saprophytes; their invasion 
of plant tissue seems to be almost a 
function of the environmental condi- 
tions, irrespective of the host genotype. 

The breeding work with sugarcane 
may be cited as contributing evidence 
that it is possible to make selections 
and obtain sugarcane types resistant to 
root rot and red rot, both necrotic 
diseases. As part of the regular screen- 
ing procedure in selection of sugarcane 
seedlings, the plants are inoculated 
with red rot and are planted in fields 
where root rot is serious. Only resistant 
plants arc considered for retention. In 
the newer sugarcane introductions, it 
is difficult to differentiate between re- 
sistance to red rot, root rot, and mosaic 
in their respective contributions to the 
success of the variety. 

Investigations by John O. Gaskill of 
the Department of Agriculture indicate 
that resistance to rotting of sugar beet 
roots by species of Botrytis and by strains 
of Phoma betae may be found among 
sugar l^cct genotypes. In this study, 
Gaskill inoculated sectors taken from 
individual sugar beet roots, represent- 
ative of various lines of breeding, with 
pure cultures of the fungi. The inocu- 
lated pieces were held in storage for 
more than 2 months under high tem- 
perature and humidity, which favor 
rotting of beet tissue. It was demon- 
strated that the sugar beet varieties 



BIECDINO POR RESIS 

differ in their resistance to decay or- 
ganisms and that individuals within 
varieties also differ significantly among 
themselves. In the inoculation tests 
only a portion of the root was used for 
indexing for keeping quality of tlie 
mother root. The remaining portion 
was thus available for planting out if 
its record warranted its use as a seed 
bearer. Progenies of roots selected for 
superior keeping quality have been 
compared for keeping quality with 
parent strains, commercial varieties, 
and with progenies from roots that 
were very subject to decay. Replicated 
tests have shown that significant ad- 
vance has been made by the selections. 

Additional evidence that resistance 
against necrotic diseases may be ob- 
tained by breeding is given by the 
investigations with sugar beet leaf spot 
[Cercospora beticola). This disease has 
l>een controlled by disease-resistance 
breeding so that the U. S. varieties 
now introduced into districts subject 
to the disease do not blight despite 
epidemics of leaf spot. With the disease, 
however, resistance manifests itself 
chiefly with the younger and maturing 
leaves, but not so strikingly with the 
older and mature foliage. In compari- 
son with susceptible varieties, the re- 
sistant plants show fewer spots on a 
leaf — often only flecks are produced — 
and the spots tend to be small and not 
confluent. Leaves of susceptible vari- 
eties become densely spotted, however, 
and shortly the leaves turn yellow and 
die. A plot of resistant plants next to 
a susceptible variety will be a green 
strip flanked by one that is dry and 
brown. The loss caused by leaf spot 
comes from the death of the leaves and 
the drain on the stored substance in 
the root brought about by the con- 
stant leaf replacement; resistant vari- 
eties retain their foliage to add to root 
substance, not to waste it. 

Mass-selection methods, so effective 
in breeding varieties resistant to curly 
top, were not adaptcci for breeding for 
resistance to leaf spot. Even under 
epidemic conditions and with inspec- 
tions throughout the season, resistant 


ANCE TO DISEASE 183 

individuals within a variety were not 
readily recognized. No immune plants 
were to be found — only degrees of 
disease involvement — ^and the individ- 
ual plants were hard to classify. Inbred 
strains of sugar beets, from earlier 
breeding work of the Department, 
were studied and of these a few (14 
out of 250) were outstanding in re- 
sistance as compared with the others. 
The pedigrees of the outstanding 
strains revealed that they had been 
subjected to several sellings as against 
mass selection operative with less re- 
sistant lines. Accordingly, the entire 
leaf spot breeding program centered 
on inbreeding as the technique to seg- 
regate genotypes with leaf spot resist- 
ance. Inbreds, as they were obtained, 
were subjected to localized epidemics 
of leaf spot in the test fields, and selec- 
tions among inbreds were made under 
conditions of drastic exposure. 

After many years of inbreeding and 
intensive selection, inbreds highly re- 
sistant to leaf spot have been obtained. 

A few of them have enough vigor to 
be desirable as breeding stock. Con- 
tinued inbreeding was found necessary 
to segregate the plants with adequate 
resistance, but vigor was lost in the 
process. The finding that hybrids made 
between certain inbred lines tended to 
regain productivity was exceedingly 
important. Although it is relatively 
easy to find hybrids that exceed in 
yield the mean of the parents, hybrids 
that exceed an open-pollinated variety 
in yield are hard to find. Very vigor- 
ous inbreds, when mated, give best 
possibilities for high productivity. Once 
highly resistant inbreds are found, the 
job resolves itself into production of a 
series of hybrids to determine the in- 
breds that give the best interactions. 

As a result of the breeding for 
resistance to leaf spot, a number of 
varieties — U. S. 217, U. S. 200 x 215, 
U. S. 215x216, and U. S. 216 x 226-^ 
have been introduced. They and the 
varieties produced by research con- 
ducted by beet sugar companies now 
occupy essentially all the sugar beet 
districts subject to leaf spot and have 



YEARBOOK OF AGRICULTURE 1953 


184 

accomplished such control that spray- 
ing or dusting against the disease is 
not necessary. The U. S. varieties in 
general are grown in Michigan, Ohio, 
Indiana, Illinois, Wisconsin, and in 
eastern Canada. Varieties of the beet 
sugar companies occupy the districts of 
Colorado, Nebraska, Wyoming, Iowa, 
and Minnesota. An outstanding in- 
bred from this research is U. S. 216, 
a high-sugar type. It is a component 
of nearly all recent U. S. hybrids. In 
the production of sugar beet hybrids, 
male sterility introduced into one 
strain is used as a device to enforce the 
hybridization with the other strain 
that supplies the pollen. The beet 
breeder, to produce male-sterile plants, 
uses a special breeding technique to 
incorporate the genetic factor that 
makes a plant fail to produce pollen. 

A HIGHLY IMPORTANT DEVEl-OPMENT 

in the breeding research with the 
sugar plants is the demonstration that 
it is possible to combine many desir- 
able characteristics in one variety. In 
a way that had been foreshadowed 
when yielding capacity, high sucrose 
quality, and disease resistance were 
incorporated in one variety. It has 
already been OvOted that sugarcane 
varieties resistant to mosaic also could 
be bred for resistance to root rot and 
red rot. In the production of such 
strains, the job re.solves itself to sorting 
out from the genotype complex those 
entities that manifest the desired 
qualities. Because of the polyploid 
nature of the sugarcane material, 
great diversity in biotypes resulted 
from the hybrids and the backcrosses 
so that there were many combinations 
from which to choose. 

With the sugar beet, the possibility 
of finding within one variety factors 
for resistance against several diseases 
was first demonstrated with U. S. 15, 
a variety selected in experiments for 
curly top resistance in New Mexico. 
It was not so resistant to curly top as 
U. S. 22, but it had moderate resist- 
ance. It also had a high resistance to 
beet rust, Uromyces betae^ and to beet 


downy mildew, Perenospora schachtii, 
Futhermore, when planted in winter 
plantings in California it was non- 
bolting — that is, it did not go to seed 
the first, or vegetative, year — in strong 
contrast to other resistant varieties 
that tended to bolt in high percentage. 
The nonbolting character and the 
resistance to rust and downy mildew, 
coupled with enough resistance to 
curly top to meet needs with early- 
planted sorts, made U. S. 15 outyield 
others with which it was compared. 

Development of the sugar beet 
industry in the Imperial Valley of 
California is to be credited to U. S. 15. 
There the sugar Ixicts arc planted in 
the fall, the crop is grown during the 
winter months, and the harvest is in 
the spring. Under such conditions 
ordinary sugar beet varieties bolt. U. S. 
33 and U. S. 34 proved to be entirely 
unsuitable. The nonbolting character 
of U. S. 15 made it especially adapted 
to the winter plantings. Its moderate 
resistance to curly top was adequate 
for the exposures to curly top that 
usually are encountered. In the San 
Joaquin Valley and coastal valleys of 
California it was possible to plant 
U. S. 15 in the winter, its nonbolting 
character again giving it advantage 
over other varieties. Winter plantings 
in coastal valleys are not usually 
subjected to curly top, but rust and 
downy mildew are serious. For those 
conditions, the resistance of U. S. 15 to 
both diseases was highly important. 

Studies have been continued in 
California to combine resistance to 
many diseases and the nonbolting 
character in one variety. Already U. S. 
56, a variety with more resistance 
to curly top than U. S. 15, has been 
released to replace the older variety. 

More recently, U. S. varieties re- 
sistant to leaf spot have been inter- 
crossed with U. S. strains resistant to 
curly top and then backcrossed. By 
repeated selections when leaf spot or 
curly top occurs, depending on the 
nature of recurrent parent in backcross, 
resistance to both diseases — essentially 



BREEDING FOR RESIS 

equivalent to that of the parents — has 
been combined in one variety. 

The same general principle of com- 
bining in a variety resistance to more 
than one disease has also been applied 
to control of sugar beet black root in 
Michigan, Ohio, and Minnesota in 
sugar beet districts where both leaf 
spot resistance and black root re- 
sistance are requisites in a successful 
sugar beet variety. Since U. S. 216, an 
inbred resistant to leaf spot, had shown 
a considerable degree of resistance to 
black root, it was possible to use leaf- 
spot-resistant synthetic varieties and 
hybrids, in which U. S. 216 is a com- 
ponent, as source varieties. In fields 
where black root is extremely severe, 
these varieties were planted. In the 
series of selections from such stocks, 
adequate leaf spot resistance has been 
retained and at the same time advance 
has been registered in resistance to 
black root. 

It has not been possible as yet to 
achieve immunity or near-immunity 
to disease in the breeding of sugar beets 
as with tobacco. Immunity from dis- 
ease must ultimately be obtained if 
losses, which arc considerable even 
with highly resistant varieties, are to 
be avoided. Growers in their enthusi- 
asm when they see the contrast be- 
tween a disease-resistant variety and 
the old susceptible type may minimize 
or overlook the lojss that disease still 
causes. Probably the varieties resistant 
to curly top are as outstanding in their 
advantages over susceptible types as 
any that could be found, short of 
immune types; nevertheless, with the 
best resistant varieties, if exposures arc 
severe, the reduction in yield may be 
as much as 25 percent of the potential 
were it possible to remove curly top 
as a factor. Similarly, varieties resistant 
to leaf spot make a profitable crop 
secure from crop failure, but the loss is 
still great over what could come from 
immunity. 

Certain wild species of Beta — no- 
tably Beta patellariSy B. procumbens, and 
B, webbiana — arc immune to both leaf 
spot and curly top. They produce Fi 


AN CE TO DISEASE 185 

seeds with Beta vulgaris that are viable, 
but the seedlings only grow a few 
inches tall and then die. By application 
of various techniques in breeding, and 
especially by use of a far wider range 
of biotypes of both the wild and culti- 
vated species, it may be possible to 
introduce into the sugar beet the genes 
for immunity to two of the most serious 
beet diseases. 

1 have cited the sugar plants as ex- 
amples of the extension of disease- 
resistance techniques for control of 
serious disea.ses. The investigations 
have required interspecies crosses when 
factors for resistance v/cre not found in 
the cultivated species Mass selection 
was effective with curly top, but 
to obtain resistance to leaf spot we 
had to inbreed repeatedly to intensify 
the factors for resistance, and then 
utilize hybrid vigor to get productive 
varieties. Male sterility has been usjed 
as a genetic tool to cnJforce hybridiza- 
tions. Because of the breadth of its 
applicability, the demonstration that 
resistance to many diseases may be 
combined in one variety is exceedingly 
important. Sugarcane, representing 
polyploids, and sugar beets, represent- 
ing heterozygous material, have been 
tailored to meet important di.scase con- 
ditions. Immunity to certain diseases 
now known for certain wild species of 
Beta offers possibilities for incorporation 
of those factors into cultivated beets. 
Those are goals for future work. 

So FAR we have considered breeding 
for disease resistance from the point of 
view of its contributions, the tech- 
niques employed in obtaining disease- 
resistant varieties, and the more recent 
developments that have broadened the 
scope of this method of di,sease control. 
These manipulations of plant material 
have been made with only meager 
understanding of the cellular mecha- 
nisms and the reactions involved. It is 
not, however, a new thing in science 
for great basic forces to be manipulated 
with only imperfect knowledge of their 
nature or even of the basic principles 
involved. In the play of biological 



YEARBOOK OF AGRICULTURE 1953 


l86 

forces, we are often even more in the 
dark than with purely physical or 
chemical phenomena. 

Our approach to the problems of 
disease resistance has had to be by trial 
and error because of the wide gaps in 
our knowledge of the physiology of 
both host and pathogen. But it may be 
of some value to summarize what we 
know of the causes of disease and to 
record the hypotheses that have been 
advanced as to the nature of disease 
resistance. 

Many plant diseases are caused 
by fungi that are known chiefly in their 
asexual stage. It has often been pos- 
sible to find the sexual stage for a par- 
ticular asexual stage, thus permitting 
classification of the pathogen as an 
Ascomycete or Basidiomycete. It is 
common for the fungus to be a patho- 
gen in its asexual stage and for fruiting 
to take place on dead tissue in what 
has been termed the “saprogenic 
phase.” Certain groups of fungi follow 
a common pattern of behavior. Thus 
the rust fungi, all obligate parasites, 
evoke a definite train of symptoms ancl 
frequently show the phenomenon of 
hetcroccism, that is, two botanically 
unrelated hosts are involved in the 
complete life cycle of the rust. Some 
forms of smuts, after a period of semi- 
commensalism — eating together, as it 
were — within the developing hosts, 
may then completely occupy the in- 
florescence. Many .smuts, however, 
show localized invasion and local ti.s.sue 
occupation. Among the plant patho- 
gens the host-parasite relations range 
from a living together, through obli- 
gate parasitism, to rather simple 
necrotic proccs.ses. A classification 
which places obligate parasitism with 
conservation of the host as of high 
order and predatism as of low order 
seems generally accepted. 

Plant pathogens may produce typi- 
cal effects of underdevelopment. 
Others, such as the Exoascaceae, cause 
definite overdevelopment. The most 
common effects produced are necrotic 
lesions, the plant as a whole respond- 


ing in various ways, depending on the 
physiological disturbance that may 
ensue. There is also generalized decay, 
which results from attack by bacteria 
or fungi on fleshy plant parts. 

Sometimes an invading fungus may 
exert pressure in its penetration of the 
plant body and in its advance from 
cell to cell. A common histological 
picture is that of a cell wall pierced 
by a strongly constricted hypha, 
which enlarges when the cell wall is 
passed. Many organisms form an 
attachment organ, called an appres- 
sorium. As invasion takes place, a 
peglike structure pas.ses into the plant. 
Through this structure the contents of 
the fungus cell flow; the penetrating 
thread enlarges to normal size or pro- 
duces haustorial, or absorbing, ap- 
paratus within the cell. The cytological 
picture as the host cells are invaded is 
a movement of the nucleus toward the 
invading thread, followed by discolora- 
tion, dissolution of the cell contents, 
and eventual death of the cell. Certain 
specialized organisms occupy the vas- 
cular tissues and cause wilting and 
death by production of toxic sub- 
stances. 

The entrance of fungi into plants 
may be direct — through the unbroken 
epidermis or at points along the lines 
of juncture of epidermal cells. The en- 
trance also may be through plant or- 
gans such as the stomata and floral 
parts. In lower types of parasiti.sm the 
entrance is through wounds. There is 
also evidence that certain organisms 
kill in advance of penetration and 
move into the disintegrating cells. 

The great German mycologist, An- 
ton de Bary, .showed in 1885 that 
Sclerotinia libertiana, the cause of a soft 
rot of vegetables, invades plants by 
means of a softening enzyme. L. R. 
Jones, in his studies at the Universities 
of Michigan and Vermont, greatly ex- 
tended our knowledge on how para- 
sites attack. He showed that Erwinia 
carotovoroy the cause of bacterial soft rot 
of carrots, cabbage, and other vege- 
tables, produces an enzyme that dis- 
solves the cementing layer between 



BREEDING FOR RESISTANCE TO DISEASE 


cells, causing tissues to lose their form 
and structure. He called the enzyme 
pectinase and distinguished it from 
those that dissolve cellulose. Mancel 
T. Munn, in his investigations at 
the Michigan Agricultural Experiment 
Station, resolved the products of the 
onion neck rot fungus, Botrytis alliiy 
into a pectinaselike enzyme and a 
toxic substance that worked together 
in rotting the onion. Such combina- 
tion effects of an enzyme that dissolves 
the pectinlike substances in cell walls 
and a plant poison have been found to 
occur commonly with necrotic dis- 
eases. 

Blackleg of potato, a bacterial dis- 
ease caused by Erwinia atroseptkay has 
been considered by some to be a form 
of the E. carotovora soft rot. John E. 
Kotila and I in our work at the Michi- 
gan Agricultural Experiment Station 
in 1925 showed that the blackleg or- 
ganism not only softens potato tissues 
but turns the cells black ahead of the 
softening. E. carotovora causes only a 
limited amount of softening and no 
blackening. Potatoes were growm in 
water culture and their uninjured roots 
were exposed to sterile filtrates from 
the blackleg bacillus cultures. The po- 
tato plants promptly turned canary 
yellow, marching the disease response 
frequently seen in diseased plants in 
the field. E. carotovora did not produce 
these effects. The two bacterial organ- 
isms therefore are alike in producing a 
softening enzyme, but only the black- 
leg organism produces tlic substance 
toxic to potato. 

It is probable that effects character- 
istic of certain plant diseases on analy- 
sis will be found to come from the joint 
acdon of enzymes and toxic substances 
produced by the pathogens. 

Death of cells is commonly attrib- 
uted to toxic substances excreted by 
the invading organism, rather than to 
general effects such as plasmolysis or 
desiccation. Stimulation and other 
types of host response have been con- 
sidered comparable to effects produced 
by minimal doses of plant poisons. The 
term “toxin” is avoided in referring to 


187 

these substances in order that analogy 
to toxins as recognized in animal pa- 
thology may not be implied. Some of 
the toxic substances are simple meta- 
bolic byproducts. L. J. Krakover, at 
the Michigan Agricultural Experiment 
Station, working with Stemphylium sar- 
ciniforme — cause of clover leaf spot — 
produced lesions on red clover leaves 
similar to those obtained by inocula- 
tions with the fungus, by pricking in 
filtrates from cultures of the organism. 
He obtained the same effects with am- 
monia solutions comparable in strength, 
as shown by Nessler reagent tests, to 
the filtrates. Erw'in F. Smith, of the 
Department of Agriculture, in seeking 
an explanation of the mechanism of 
crown gall formation, attributed the 
effects produced by Agrohacterium turne- 
faciens to acetic acid and other chemi- 
cals arising from the growth of the bac- 
terium. Smith suggested that they in- 
cite excessive cell division or remove 
natural inhibitors to growth of cells, 
which, in the normal plant, maintain 
a balance in cell division. He sup- 
ported his hypothesis by experimental 
production of plant tumors by com- 
parable chemical treatments in ab- 
sence of the organism. 

The vascular diseases caused by the 
Fusaria have received a great deal of 
study. It is characteristic of these dis- 
eases that the shoots of affected plants 
wilt. At first it was assumed that w'ilt- 
ing came from plugging of the water- 
conducting vessels, comparable to 
what Erwin F. Smith had demon- 
strated with the bacterial wilt of cu- 
curbits. R. W. Goss, at the Michigan 
Agricultural Experiment Station, dem- 
onstrated that if cut ends of potato 
shoots were immersed in filtrates of 
Fusarium oxysporum, the shoots prompt- 
ly wilted. E. W. Bran^es at Cornell 
University demonstrated the same 
thing for Fusarium cubense with buck- 
wheat, bean, and banana leaves. That 
wilting comes from mechanical block- 
ing was thus brought into question. 
Filtrates of ordinary saprophytes were 
shown to produce wilting. But the asso- 
ciation of wilting with a specific Fu~ 



YEARBOOK OF AORfCULTURE 1953 


1 88 

sarium is significant, because it enters 
the plant and there produces its toxic 
substances. More recently, Ernst Gau- 
mann in Switzerland has extensively 
developed this field of research and 
attetnpted to relate it to antibiosis. 

Plant pathologists, without exclud- 
ing the possil)ility of complex organic 
compounds being involved, incline to- 
ward the point of view that the more 
simple' compounds which are byprod- 
ucts of fungus metabolism should first 
be explored in the study of toxic ef- 
fects of fungi on host cells. Blocks in 
the respiration cycle arc receiving at- 
tention as probable factors in disease 
production. 

The defense mechanisms, if they may 
be so called, that are invoked in plants 
by infectious organisms appear to be 
relatively simple; the invader may in 
some ca.ses stimulate the formation of 
corky ti.s.sue.s or may produce other 
growth eflects. Automatic walling-ofi‘ 
of the parasite occurs with hypersen- 
sitive varictie.?, in which invasion of 
the fungus is accompanied by prompt 
collapse of tissue. Thus the paradoxical 
situation exists in which hypersensi- 
tivity limits fungus extension to a mere 
flecking, and the most su.sceptible 
plants arc the most resistant. 

It might seem simplest to explain 
disease resistance as an antagonism of 
the juices of the host cell to the para- 
sitic invader. But antagonistic chem- 
icals of host plants have mostly re- 
mained undiscovered — except for the 
catechol and protocatechuic acid in 
the red- and brown-skinned onions 
resistant to Colletotrichum circinans and 
Botrytis alliiy as found by K. P. Link 
and J. C. Walker in their investiga- 
tions at the University of Wisconsin in 
1933. These chemicals arc not found 
in the su.sceptible white-skinned types. 
But the chemical antagonism found by 
Link and Walker did not exienrd to all 
fungi, since Aspergillus niger could grow 
in extracts from the pigmented onions. 

Plants, dej)ending on how they are 
grown, vary in their resistance and in 
their chemical make-up. Furthermore, 


it is difficult to determine biochemical 
compounds, and so far there are no 
leads to relate any compound or class 
of compounds to disease resistance. 
Because of such obstacles, this field of 
research has remained almost unex- 
plored. In the two decades since the 
discovery of resistance in pigmented 
onions, and the detennination of the 
chemicals responsible, no additional 
information of parallel exactness has 
been obtained. 

There is no accepted evidence of the 
production of antibodies in plants as a 
result of invasion by fungi. Localiza- 
tion in fungus attack, absence in plants 
of any circulating medium comparable 
to the blood stream, and the appar- 
ently simple chemical reactions in- 
volved in many of the lethal effects of 
fungi upon plant cells may explain 
w'hy infected plants do not produce 
protective substances comparable to 
those known in animals. 

Even with the bacterial organism, 
A. tumejaciens^ which (on the basis of 
the type of ceil division, presence of 
tumor strands, and the profound dis- 
turbance.s in cell morphogenesis) Er- 
w'iii F. Smith considered as producing 
a plant cancer, it was not po.s.sible to 
demonstrate antibody formation. In 
Smith’s tests, repeated inoculations of 
an individual plant did not bring 
about lessening of reaction. 

The virus disease curly top furnishes 
some evidence of the production of 
protective substances as a result of in- 
fection. 'Lhis effect differs from .satura- 
tion phenomena known for tobacco 
ring spot and interference as known to 
take place between viruses. J. M. 
Wallace, working with curly top in 
the Department of Agriculture in 
1944, inoculated tobacco and tomato 
plants by means of l>eet leaf hoppers, 
Circulijera tenellus, which had fed on in- 
fected plants. The inoculated plants 
soon showed severe symptoms of curly 
top. Tomato plants did not recover, 
but many tobacco plants did. The 
leaves from such plants showed only 
mild symptoms in a few weeks. If 
healthy shoots of tobacco or tomato 



BREEDING FOR RESISTANCE TO DISEASE 


were grafted onto the tobacco plants 
that showed recovery, only mild symp- 
toms were product on the scions. 
The virus in the recovered plants was 
demonstrated to be highly virulent by 
insect transmission tests, but the 
symptoms produced were mild. When 
cuttings were taken from recovered 
plants of tobacco, or from tomato 
plants protected by the scion grafts 
from recovered tobacco plants, and 
these in turn were grafted onto tomato 
plants, the disease produced on the 
tomato was mild. Wallace attributed 
these results to protective sul)stances 
against curly top developed in the re- 
spective tobacco and tomato plants in 
the course of infection. Once a tomato 
plant is protected, serial grafts give 
only a succession of mild curly top re- 
actions, the scions taken from such 
plants serving as donors of the protec- 
tive substance. If protective substances 
were to be sought, it would be logical 
to look for them with a systemic disease 
such as curly top. 

We have considered so far entrance 
of fungi into plants, the mechanism 
of disease production, and the re- 
sponses provoked. For the higher 
grades of parasitism, certain other 
essential requirements in disease pro- 
duction, namely, conditions for estab- 
lishment of the organism within the 
host, are important. Suggestions have 
been made as to osmotic differentials 
which must prevail in host-parasite 
relationships. According to this, the 
concentration of the cell sap of the par- 
asite must be higher than that of the 
crop plant, if the parasite is to take 
water from the host. No regularity 
in this respect has been found. To 
explain resistance of ^ea to Puccinia 
sorghi, the hypothesis was advanced 
that susceptibility is determinetl by 
the presence of a relatively large 
quantity of a nutritive substance that 
attracts the fungus after penetration 
and makes possible abundant develop- 
ment of the rust. In resistant host 
plants, this hypothetical substance is 
present only in very small quantity; 
hence the fungus dies of starvation, 


189 

and that, in turn, leads to necrosis of 
the host tissue. It has been suggested 
that the slower development of the 
pathogen in a resistant host, as com- 
pared with abundant growth in a 
susceptible host, may be ascribed to a 
more favorable nutritive substance 
ratio in the latter. 

In some investigations at the Michi- 
gan Agricultural Experiment Station, 
conducted with L. J. KJotz, I studied 
selectivity for host plants of certain 
species of Cercospora. Cercospora apii 
causes lesions only on celery and 
closely related umbellifers. It does not 
attack plants from other families. 
Diseased celery leaves gave strong 
nitrite tests, as compared with absence 
of nitrite in healthy leaves, indicating 
that breakdown of proteins and other 
nitrogenous compounds occurred. 
Now any of the species of Cercospora 
will grow readily and indiscriminately 
on cooked plant tissue, but with 
respect to living tissue they are very 
choicy. Furthermore, the pathogenic 
limits apparently tend to disappear as 
leaf tissue matures and cells are about 
to die. It does not seem that a parasite 
such as Cercospora, once it invades the 
cells, should have difficulty in appro- 
priating water necessary for its growth. 
It seems probable also that the car- 
bohydrates, such as sucrose, dextrose, 
or other sugars, and water-soluble 
substances exist in plants in much the 
same available state as in synthetic or 
cooked media. As a result of our 
investigations, we suggested that the 
nitrogen phase of fungus nutrition was 
particularly important and that the 
protein-dissolving enzymes that a 
given fungus has may determine its 
capacity for attacking a given plant 
species. 

The other side of the picture also 
demands attention. How may the 
nutrition of the host influence sus- 
ceptibility to disease? The factors that 
predispose a plant to disease have long 
been considered in phytopathology, 
but there has been more speculation 
than actual research. A well -nourished 



YEARBOOK O F A O R I C U L T U R E 1953 


190 

plant might well be expected to 
withstand disease better than one that 
is undernourished. But heavily fer- 
tilized plots frequently show more 
disease than untreated plots. A plau- 
sible explanation is that with heavier 
plant growth the conditions favor 
infection and the disease situation is 
aggravated. The literature is replete 
with experiences that seem to operate 
in reverse of anticipated results. For a 
number of virus diseases it has been 
shown recently that the predominating 
effect of added nutrients is to increase 
susceptibility. 

Investigations in 1937 by F. G. 
Larmer, of the Department of Agri- 
culture, on the phoma root rot of 
sugar beets gave some interesting 
results. Phoma hetae, the causal or- 
ganism, occurs rather generally in 
sugar beet tissue. The fungus is 
apparently seed-borne, and young 
plants may be killed by severe attack. 
Under ordinary conditions it appears 
to live in the beet plant without pro- 
voking any visible symptoms. If the 
sugar beet lags in its growth because 
of drought or lack of soil nutrients, the 
fungus may produce extensive and 
conspicuous decay of tissue. In the 
fall the fungus causes serious rotting 
of sugar beets in storage piles. Larmer 
found that well-nourished sugar beets, 
grown with adequate water supply, 
kept better in storage than sugar 
beets from tne check plots that suffered 
from drought and poor soil conditions. 
The effects of plant food elements, 
especially phosphorus, were decisive. 
Sugar beets, grown with adequate 
phosphate, showed minimal decay as 
compared with serious decay in the 
sugar beets not receiving additional 
phosphorus. Inoculation tests were 
made with Phoma hetae and showed 
that the plants grown with adequate 
phosphate definitely resisted invasion 
by the fungus, whereas the sugar beet 
roots grown with limited phosphate 
decayed. The practical applications 
of the experiments are obvious and 
afford an accessory advantage, besides 
erbp increase, to be dcriv^ from 


appropriate applications of fertilizer. 
But the point to be stressed is that it 
seems definitely to be shown that 
plants whose nutrition was adequate, 
especially with respect to phosphate, 
were more resistant to Phoma betae. In 
recent experiments, adequate nitrogen 
nutrition has given improved keeping 
quality to sugar beets. 

Black root of sugar beet has been 
found to l^e a disease complex, con- 
sisting of an acute form caused by the 
ordinary damping-off organisms and 
a chronic form caused by Aphanomyces 
cochlioides. Black root is most serious on 
soils of low available phosphate. Poor 
stands that result from attack of the 
soil organisms may be prevented by 
proper phosphate fertilization. A plau- 
sible explanation of the better stands 
that accompany liberal applications of 
P3O5 fertilizer is that phosphate makes 
the young sugar beet plants more re- 
sistant to the pathogens. The effect 
must be upon the beet, because abun- 
dant trial has failed to indicate any 
effects upon the fungi from comparable 
phosphate feeding. Black root control 
now is based on two things: First, the 
production of varieties that combine 
resistance to black root and to leaf 
spot; second, proper fertilization of 
the resistant variety with PaOg and 
other necessary food elements. A fa- 
vorable environment for the sugar beet 
increases the advantages that come 
from disease resistance. 

The associative effects of nutrition 
and disease are not unique in phyto- 
pathology, Pronounced fungus inva- 
sion occurs with certain deficiency dis- 
eases — for example, the fungus rots or 
severe outbreaks of mildew that occur 
on plants short of boron. Existence of 
fungus disease does not of itself indicate 
malnutrition, but the association of 
disease and poor plant feeding is fre- 
quent. Sugar beet plants growing with 
scant phosphate show severe leaf in- 
jury following spotting of the leaves by 
Cercospora beticolay not because the leaf 
spot fungus is more aggressive but 
l^cause weak parasites such as Alter* 
naria can now invade and greatly en- 



BREEDING FOR RESISTANCE TO DISEASE 


large the cercospora spots. The sec- 
ondary attacks following leaf spot of 
this type do not occur with well-nour- 
ished plants. With many important 
plant diseases, the host-parasite rela- 
tionships are very delicately balanced, 
and proper feeding of the crop plant 
may be highly important.. 

The nature of disease causation and 
of plant response may at first glance 
seem to contribute little to the prac- 
tical problems of disease-resistance 
breeding. These seem to be moving 
along, with little reference to the the- 
ory. It is not unusual for practical 
applications to go far beyond the sci- 
entific explanation of phenomena — 
just as a driver may run a machine 
without understanding the mechanism 
or the source of pow'er. But alw’ays in 
science the theoretical basis is the 
fruitful source of new concepts and 
new approaches. 

We are concerned with the ways and 
means of the pathogen’s attack and of 
the host plant’s defense. With hun- 
dreds of thousands of pathogens, each 
composed of numerous biotypes, we 
cannot expect a single pattern of dis- 
ease production or to find some specific 
substance that confers resistance. Thus, 
the theory is important in teaching 
what not ro search for. The theory is 
important also in its teachings with 
regard to plant reactions. As I have 
brought out, di.sease exposure must be 
maximum, and knov/ledge of plant 
pathology must be put into service to 
initiate infection, produce disease, and 
finally to classify affected plants. The 
theory teaches also the specificity and 
delicateness of the interactions that 
exist between host and parasite, and 
the play of environmental factors as 
they influence these reactions. Thus 
light, temperature, length of day, nu- 
trition, and the entire range of physio- 
logical forces arc concerned. Plant 
response when subject to the parasite 
is the sum total of these effects. The 
goal of breeding for disease resistance 
is to manipulate host and parasite and 
the factors playing upon them in such 
a way that for a given environment 


19 * 

disease-safe varieties may be provided. 

The fungi, bacteria, and the viruses, 
each in its own specific manner, es- 
tablish a food and water relation with 
the host plant. The invader may be 
tolerated; it may dwarf the plant or 
cause overgrowths or decay; it may 
kill the host. The tools the invader uses 
are enzymes and toxic substances; in 
considerable part, mere occupancy of 
the cells, appropriation of food, and 
the effects of the metabolic byproducts 
from the growth of the pathogen may 
be sufficient to explain the disease 
signs and symptoms. 

Disease resistance as considered here 
is that which is of protoplasmic origin, 
as opposed to mechanical walling-off, 
escape, or other reactions than purely 
vital ones. We do not know at all 
what makes one plant less susceptible 
than another, nor do w'e know the 
basis of fungus specificity that makes 
one species, genus, or family of plants 
completely immune from a given para- 
site. Of the two hypotheses commonly 
advanced, antagonism (presumably 
chemical) and the so-called starvation 
hypothesis, the latter seems most ten- 
able, with nitrogen nutrition appearing 
to be a significant phase. 

Despite the common question posed 
by farmers, we do not know how to 
feed plants so as to make them more 
disease-resistant. Until recently, re- 
search along this line has been neg- 
lected. There are also strong indications 
that disease-resistant varieties may be 
made to do better by proper nutrition. 
The next decade may see important 
developments as the nutrition of our 
crop plants is studied from the point 
of view of plant disease control. 

George H. Ooom was loaned in 1^24 
and in Jp2j to the Bureau of Plant Industry, 
Soils, and Agricultural Engineering by the 
Michigan State College, where he was pro- 
fessor of botany, to initiate the research pro- 
gram of breeding varieties of sugar beet re- 
sistant to curly top and to leaf spot. After 
his return to the College, he continued as 
plant pathologist on a half-time basis with 
the Bureau until tgsg, when he left Mich- 



YEARBOOK OF AGRICULTURE 1953 


192 

igan State College to become principal pa- 
thologist in charge of sugar beet research 
projects in the division of sugar plant inves- 
tigations. He received his undergraduate 
training at the University of Illinois^ his 
master^s training at the University of 
Nebraska^ and his doctorate at the Univer- 
sity of Michigan. After directing the sugar 
beet project of the Bureau for 23 years, he 
left administrative work to conduct research, 
particularly on virus yellows. 

For further reading: 

Breeding disease resistant varieties: 

W. A. Orton: The Development of Farm 
Crops Resistant to Disease, U. S. D. A. Tear- 
hook for i()o8, pages 433-464. 

L. R. Jor^es^ J. C. Walker, and IV. If. Tisdale. 
Fusarium Resistant Cabbage, JVisconsin Agri- 
cultural Experiment Station Research Bulletin 48, 
34 pages, rgso. 

H. L. Bolley: Flax Wilt and Flax-sick Soil, 
North Dakota Agricultural Experiment Station 
Bulletin 30, 38 pages, iQor. 

G. H. Coons: Some Aspects of the Fusarium 
Problem in Plant Pathology and Physiology 
in Relation to Man, Mayo foundation Lectures 
of !gs6~-2j, pages 43-32. W. B, .launders Com- 
pany, Philadelphia, 1328. 

Variability of fungi: 

E. C. Stakman: Plant Pathologists* Merry- 
Go-Round, (f Heredity, volume 37, pages 

233-263, 1346. 

W. B. Brierley: The Relation of Plant Pa- 
tliology to Genetics, Report of Imperial Botani- 
cal Congress (London) for i 32. p pages 111-124, 
University Press, Cambridge. 

Nature of disease resistance: 

K. P. Link and J. C. Walker: The Isolation of 
Catechol from Pigmented Onion Scales and 
Its Significance in Relation to Disease Re- 
sistance in Onions, Journal of Biological Chem- 
istry, volume 100, pages 373-383, ^333. 

8.T Hiker: The Relation of Some Chemical 
and PhysioclK mical Factors to the Initiation 
of Pathological Plant Growth, Growth IV 
{Supplement}, pages 103-117, 1342. 

E. C. Stakman: Relation Between Puccinia 
graminis and Plants Highly Resistant to Its 
Attack, Jmrnal of Agricultural Research, volume 
4, pages 133-200, 1313. 

J. M. Wallace: Acquired Immunity from 
Curly Top in 'fobacco and 'I'omato, Journal 
of Agricultural Research, volume 63, pages 187- 
214, 1344. 

Nutntional aspects: 

F. C. Bawden and B. Kassanis: Some Effects 
of Host Nutrition on the Susceptibility of 
Plants to Infection by Certain Vii*uses, Annals 
of Applied Biology, volume 37, pages 46-47, 1330. 

F. G. Ijsfmer: Keeping Quality of Sugar 
Beets as Influenced by Growth and Nutri- 
tional Factors, Jowrial of Agricultural Research, 
volume 34, pages 183-138, 1337. 


Some Sources of 
Resistance in 
Crop Plants 

Frederick J. Stevenson, Henry A. Jones 

We give in this chapter a list of crop 
plants and the diseases to which resist- 
ance has been found. Listed in order 
are the name of the crop, the disease 
(with the causative organism), the 
original source of resistance (O. S. R.), 
the present source of resistance (P. S. 
R.), and the mode of inheritance. 

The abbreviation C. I. stands for 
Cereal Investigations — an accession 
number that, like a name, is assigned 
to a new variety of cereal grain. 

In many instances the inheritance is 
reported as unknown cr undetermined. 
Often the breeding behavior of the 
character for resistance is known, but 
the exact number of genes imolved 
has not been determined. Some of the 
factors that make it hard to give a 
definite genetic explanation are mul- 
tiple genes, polyploidy, physiologic 
races of causative organisms, and the 
effects of environment. 

Write to your State agricultural 
experiment station for information 
concerning the sources of plant ma- 
terials listed in this chapter. 

For many diseases of crop plants 
there is still no known source of resist- 
ance. The search, however, is going 
forward steadily. If crop failures are 
to I^c avoided, new sources and higher 
levels of resistance to many destructive 
diseases have to be located. Undoubt- 
edly many new sources of resistance 
will be uncovered in wild and culti- 
vated species to be added to this 
already imposing list. 



SOME SOURCES OF RESISTANCE IN CROP PLANTS 


CEREALS: barley, com, oats, rice, 
wheat. 

Barley 

Covered smut, Ustilago ho^dei. O. S. 
R.: Jet C. I. 967, Ogalitsu C. I. 7152, 
Anoidium C. I. 7269, Kura C. I. 4306, 
Suchow C. I, 5091, Apsheron C. I. 
5557, Hokudo C. I. 51 76, C. I. 4308-2, 
and others in the barley world collec- 
tion. P. S. R.: Same as preceding. 
Inheritance: Undetermined. Inherit- 
ance studies are difficult because it 
is hard to get consistently high levels 
of infection. 

Leaf rust, Pticcinia hordei. O. S. R.: 
Bolivia C, I. 1237 and 100 to 200 
additional varieties in the barley 
wwld collection. P. S. R.: Commercial 
varieties having leaf rust resistance 
often derived this resistance from the 
sources in O. S. R. Inheritance; Mon- 
ogenic dominant in most crosses; an 
additional gene may be present in 
some varieties. 

Loose smut, Ustilago nigra. O. S. R.: 
Pannier C. I. 1330, Jet C. 1 . 967, 
Anoidium C. I. 7269, Ogalitsu C. I. 
7152, Kura C. I. 4306, Suchow C. I. 
5091, Hokudo C. I. 5176, Apsheron 
C. I. 5557 . I- 4308-2, 4326-*, 
4327, 4329, 4967, and others in the 
world collection. P. S. R.; Same as 
preceding. Inheritance; Undeter- 
mined. Inheritance studies are diffi- 
cult because it is hard to get consistent- 
ly high levels of infection. 

Loose smut, Ustilago nuda. O, S. R.: 
Jet C. I. 967, Trebi C. I. 936, Valen- 
tine C. I. 7242, Ogalitsu C, I. 7152, 
Anoidium C. I. 7269, Abyssinian C. 1 . 
668, Bifarb C. 1 . 3951-3, Kitchin C. 
I. 1296, Afghanistan C. I. 4173, Su- 
chow C. I. 5091, 0 . I. 4966, a number 
of hooded winter barley selections 
from Tennessee Beardless and Mis- 
souri Early Beardless, and other varie- 
ties in the world collection. P. S. R.: 
Resistant genes from some of these 
varieties now have been transferred 
to commercial varieties; for example, 
Velvon, Titan, Tregal, etc. Inherit- 
ance: Four resistant genes have been 
identified so far, two of which are 


>93 

dominant and two are intermediate 
in effect. The smut gene in Valentine 
is closely linked with the gene for stem 
rust resistance. 

Net blotch, Pyrenophora teres. O. S. 
R.: Canadian Lake Shore C. I. 2750, 
Tifang C. I. 4407-1, Manchu C. I. 
4795, Ming C. 1 . 4797, Harbin C. I. 
4929, Velvet 26-95, ^^id 

about 70 additional varieties in the 
barley world collection. P. S. R.: Par- 
tial protection to this disease now is 
present in some ccmmercial varieties, 
which derived theS germ plasm from 
barleys coming from Manchuria. 
Inheritance: Undetermined: 

Powdery mildew , Erysiphe graminis. 
O. S. R.: Duplex C. I. 2433, Hanna 
C. I. 906, Goldfoil C. I. 928, Arlington 
Awnless C. I. 702, Chinermc C. I. 
1079, Algerian C. I. 1179, Kwan C. I. 
1016, Psaknon C. I. 6305, Monte 
Cristo C. 1 . 1017, West China G. I. 
7556, and many other varieties in the 
barley world collection. P. S. R.: Re- 
sistance to this disease is now found 
in several commercial varieties; for 
example. Atlas 46 and Eric. Inherit- 
ance: Nine dominant or incompletely 
dominant and three recessive genes 
for reaction to race 3 have been lo- 
cated. 

Scab, Gibbet elia O. S. R.: Chev- 
ron C. 1 . iiii, Himalaya C. I. 2448, 
Korsbyg C. I. 918, Cross C. I. Nos. 
1613 and 2492, Peatland C. I. 5267, 
Svansota C. I. '907, and Golden 
Pheasant C. I. 2488. P. S. R.: Same 
as preceding. Inheritance: Undeter- 
mined. 

Scald, Rhynchosporium sccalis. O. S. 
R.: Turk C. 1 . 5611-2, La Mesita 
C. I. 7565, Modoc C. I. 7566, Trebi 
C. I. 936, and a number of additional 
varieties in the barley world collec- 
tion. P. S. R.: Resistance to scald is 
present in Atlas 46, which was derived 
from the variety Turk. 

Spot blotch, Helminthosporium sati- 
vum. O. S. R.. Oderbruckcr C. I. 4666, 
Peatland C. I. 5267, Chevron C. 1 . 
IIII, Jet C. I. 967, OAC 21 C. I. 
1470, Persicum C. I. 6531, Brachytic 
C. I. 6572, and others in the barley 



YEARBOOK OP AGRICULTURE 1953 


194 

world collection. P. S. R.: The resist- 
ance in Moore is an example of the 
transfer of resistance from Chevron/ 
Olli. 

SiTSM RUST, Puccinia graminis. O. S. 
R.: Chevron C. I. iiii, Pcatland C. 
I. 5267, Hietpas 5 C. I. 7124, Kindred 
C. I. 6969, and about 50 additional 
varieties in the barley world collection. 
P. S. R.; Several commercial varieties; 
for example, Mars, Moore, Kindred 
Feebar, and Plains. Inheritance: Mon- 
ogenic dominanr;^n additional gene 
may be pre.scnt in some varieties. 

Stripe, Helminthosporium gramineum. 
O. S. R.: Hannchen C. I. 531, Trebi 
C. I. 936, Club Mariout C. I. q6i, 
Persicum C. I. 6531, Brachylic C. I. 
6572, Lion 923, and others in the bar- 
ley world collection. P. S. R.: Same as 
preceding. Inheritance: Six or more 
separate genes are involved and various 
degrees of dominance have been en- 
countered. 

Corn 

Brown spot, Physoderma zecLe-maydis. 
O. S. R.: L87 and L87-2. P. S. R.: 
Same as preceding. Inheritance: Un- 
known; probably polygenic. 

Diplodia ear rot, Diplodia zeae, 
O. S. R.: R4, C. 1 . 540, III. 90, P. S. 
R.: Same as preceding. Inheritance: 
Unknown; probably polygenic. 
Diplodia stai r rot, Diplodia Zfae, 

O. S. R.: B14, B15, I159, K166, 

K201, C103. P. S. R.: Same as preced- 
ing. Inheritance: Unknown; probably 
polygenic. 

Gibberella stalk rot, Gibberella 
zeae. O. S. R.: C. I. 21E, K201, T8. 

P. S. R.: Same as preceding. Inherit- 
ance: Unknown; probably polygenic. 

Helminthosporium leaf spot, Hel- 
minthosporium carbonum, race i. O. S. 
R.: Most inbred lines arc resistant. 
Susceptible inbred lines are: Pr, K61, 
Mo. 2 1 A, K44. P. S. R.: Same as 
preceding. Inheritance: Monogenic 
dominant. 

Northern corn leaf blight, Hel- 
minthosporium iurcicu?n, O. S. R.: Mo. 
2 1 A, NC34, L97, Ky. 114, T13, C. 1 . 
23 » G103, Ky. 36-11, K175, K148, 


and R39. P. S. R.: Same as preceding. 
I nheri tance : Polygenic . 

Rust, Puccinia polysora, O. S. R.: 
Little critical information available 
but in a greenhouse test inoculated 
when 6 weeks old, the following lines 
were resistant to infection: Hy, W22, 
461-3, 38-1 1, Ohio 07, K148, T14, and 
C. I. 15. P. S. R.: Same as preceding. 
Inheritance: Unknown; probably poly- 
genic. 

Rust, Puccinia sorghi. O. S. R.: Little 
critical information available but in- 
bred lines WF9, B2, C. I. 540, I. T. E. 
701, and 111. 90 have shown some re- 
sistance under field conditions. P. S. 
R.: Same as preceding. Inheritance: 
Probable polygenic; resistance to phys- 
iologic form 3, a monogenic dominant. 

Seedijng blight, Pythium species. 
O. S. R.: W23. P. S. R.: Same as 
preceding. Inheritance: Unknown; 
probably polygenic. 

Seedling bught, Penicillium oxali- 
cum. O. S. R.: III. 90, W22, Ohio 41, 
38-11, W24, A375. P. S. R.: Same as 
preceding. Inheritance: Unknown; 
probably polygenic. 

Smut, Vstilago maydis, O. S. R.: Ind. 
33“i6, A321, and JK. P. S. R.: Same 
as pjreceding. Inheritance: Polygenic. 

Southern corn leaf blight, Hel- 
minthosporium maydis. O. S. R.: CT03, 
Tr, G, Mi 4, 0426, and W20. P. S. R.: 
Same as preceding. Inheritance: Un- 
known; probably polygenic. 

Stewart’s wii.t (late infection or 
leaf-blight phase), Bacterium slewartii. 

O. S. R.: K4, Ky. 27, C103, Ohio 28. 

P. S. R.: Same as preceding. Inherit- 
ance: Not fully known. Systemic p)hase 
apparently controlled by two major 
and one minor gene. 

Oats 

Anthracnose, Colletotrichum grami- 
nicola. P. S. R.: Early Red Rustproof, 
Red Rustproof, Saia, and Victoria. 
Inheritance: Unknown. 

Bacterial stripe blight, Pseudo- 
monas striajaciens. P. S. R.: Aurora, 
Coastblack, Golburt, Culberson, Ful- 
ghum, Navarro, Red Rustproof, Ru- 



SOME SOURCES OF RESISTANCE IN CROP PLANTS 


akura, Swedish Select, and Winter 
Turf. Inheritance: Unknown. 

Black loose smut, Usiilago avenae, 
P. S. R.: Black Mesdag, Bond, Land- 
hafer, Markton, Navarro, Victoria, 
and many derivatives of preceding. 
Inheritance: Monogenic to polygenic. 

Covered smut, Ustilago kolleri, 
P. S. R.: Black Mesdag, Bond, Land- 
hafer, Markton, Navarro, Victoria, 
and many derivatives of preceding. 
Inheritance: Monogenic to polygenic. 

Crown rust, Puccinia coronata avenae. 
P. S. R.: Arkansas 674, Bond, Bondvic, 
Landhafer, Santa Fe, Trispernia, 
Ukraine, Victoria, and many deriva- 
tives of preceding. Inheritance: Mono- 
genic to polygenic. 

Halo blight. Pseudomonas corona- 
faciens. P. S. R.: Buck 212, Clinton, 
Coastblack, La Estanzuela, La Pre- 
vision, Navarro, Quincy Red, and 
Victoria. Inheritance: Unknown. 

Helminthosporium leaf blotch, 
Pyrenophora avenae, Helminthosporium 
avenae, P. S. R.: Coker’s B 1-47-67, 
Coker’s Bi~47-79, and Wis. X279-1. 
Inheritance: Unknown. 

Mosaic, Marmor terrestre var. typicum, 
P. S. R.: Fulghum and Red Rust- 
proof. Inheritance: Unknown. 

Oat blast (not parasitic). P. S. R.: 
Alaska, Eagle, Fulghum, Hatchett, 
Kanota, and Lasalle. Inheritance: 
Unknown. 

Pov\T5ERY mildew, Erysiphe graminis 
avenae, P. S. R.: Missouri 0-205, Mis- 
souri 04015, Neosho X Landhafer, Red 
Rustproof X Victoria Richland C. I. 
4386, Sandhafer, and White Mildew- 
Resistant. Inheritance: Trigenic. 

Pythium root necrosis, Pythium 
debaryanum. P. S. R.: Black Algerian, 
Coastblack, Early Red Rustproof, 
Flughafer, Red Algerian, and Rua- 
kura. Inheritance: Unknown. 

Red leaf (yellow dwarf), virus, 
no scientific name. P. S. R.: Anthony- 
Bond X Boone (C. 1 . 5220), Anthony- 
Bond X Boone (G. I. 5218), Anthony- 
Bond X Boone (C. 1. 5224), Arkwin, 
Arlington, Atlantic, Bondvic (C. I. 
5401), Fulghum 708, Fulwin, and 
Mustang. Inheritance: Unknown. 


195 

Septoria leaf spot and black 
stem, Leptosphaeria avenaria, P. S. R.: 
Anthony-Bond x Boone, Ajax, Beaver, 
Branch, Clintafe, Clinton, Shelby, 
Spooner. Inheritance: Unknown. 

Stem rust, Puccinia graminis avenae, 
P. S. R.: Canuck (Hajira x Joanette), 
Clinton x Ukraine (C. I. 5871), Joan- 
ette strain, Richland, Victoria x 
(Hajira x Banner), White Tartar, and 
many derivatives of preceding. In- 
heritance : Monogen ic . 

Victoria blight, Helminthosporium 
victoriae, P. S. R.: Most varieties, 
except crown-rust-resistant Victoria 
derivatives. Inheritance: Monogenic. 

Rice 

Blast, Piricularia oryzae. O. S. R.: 
Collections by the United States De- 
partment of Agriculture P. E. I. 13056 
(C. I. 1344) from Formosa in 1905; 
P. E. I. 31169 (C. I. 1779) from the 
Philippines in 1911; and selections 
from commercial variety in the United 
States. P. S. R.: Available now as 
Zenith and Rexoro. Inheritance: 
Monogenic dominant (Sasaki). 

Brow^n leaf spot, Helminthosporium 
oryzae. O. S. R.: Collection of United 
States Department of Agriculture C. 1 . 
5309 from China in 1918 and local 
selection. P. S. R.: Available now as 
commercial types being developed. 
Inheritance: Polygenic. 

Narrow brown leaf spot, Cerco- 
spora oryzae. O. S. R.: Collection by 
United States Department of Agri- 
culture C. I. 461 and C. I. 654 from 
1904 exhibit of Philippine Islands at 
St. Louis, Mo. P. S. R.: Kamrosc and 
Asahi. Inheritance: Monogenic dom- 
inant (Rykcr, Jodon). 

Wheat 

Black chaff, Xanthomonas translucens 
undulosum. O. S, R.: Unknown. P. S. 
R.: Thatcher and Marquis. Inherit- 
ance: Unknown. 

Bunt, Tilletia spp. O. S. R.: Un- 
known. P. S. R.: Common bunt: 
Brevor, Elmar, Hope, Hussar, Mar- 



YEARBOOK OF AGRICULTURE 19S3 


196 

tin, Ncwthatch, Oro-Turkey-Florcnce, 
Rex-Rio, and White Federation 58. 
Dwarf hunt: Brevor, Elmar, Hussar, 
Martin, Wasatch, Inheritance; Mono- 
genic to polygenic. 

Flag SMUT, Urocystis tritivi, O. S. R.: 
Unknown. P. S. R.: Golden. Inherit- 
ance; Unknown. 

Leaf rust, Pwmnifl ruhigo-vcra tritici , 
O. S. R.: Unknown. P. S. R.: Com- 
mon wheat: Exchange, Frontana from 
Brazil (III. i x Chinese)* — Timopheevi 
(Wis. 245), Klein Titan, La Prevision 
25 (P. E. I. 168732 from Argentina), 
No. 43 (P. E. 1 . 159106 from Union of 
South Africa), Rio Negro (P. E.. 1 . 
168687 from Brazil), Supremo, and 
Timstcin. Durum wheal: Bcladi (P. 
E. 1 . 57662-3 from Portugal), Golden 
Ball-Iumillo-Mindum, RL 1714, 
Tremez Mollc (P. E. I. 56258-1 from 
Portug.al), Tremez Rijo (P. E. I. 
56257-1 from Portugal). Inheritance: 
Monogenic to polygenic. 

Loo.SE SMUT, IJstilago tritici, O. S. R.: 
Unknown. P. S. R,: Hope, Kawvale, 
Leap, Pawnee, and Trumbull. Inher- 
itance: Trigenic and unlinown. 

Pow'DERY Mii.DEW, Erysiphc gramims, 
O. S. R.: Unknown. P. S, R.: A.sosan 
(P. E. I. 155256 from Japan), Indian, 
Michigan Amber selection, Picard ic 
(P. E. I. 168670 from France), Prog- 
ress, Sturgeon, Suwon 92 (P. E. I. 
157603 from Korea), Trumbull-Red 
Wonder-Stcintim, C. I. 12559. disher- 
itance: Unknown!. 

Speckled i.eaf spot, Septoria tritici. 
O. S. R.: Unknown. P. S. R.; Glad- 
den, Nabob, and Nured. Inheritance: 
Unknown. 

Stem rust, Puccinia gratninis tritici. 

0. S. R.: Unknown. P. S. R.: Com- 
mon wheat: Egypt Na-95 (P- d. 
153780), Hope, Kenlana, Kenya 58, 
Kenya 1 1 7A, McMurachy, No. 43 
(P. E. I. 159106 from Union of South 
Africa), Red Egyptian, Thatcher, 
Timstein. Durum wheat: Beladi (P. E. 

1. 57662-5 from Portugal), Golden 
Ball-Iumilio-Mindum, RL 1714, 
Tremez Mollc (P. E. I. 56258-1 from 
Portugal), Tremez Rijo (P. E. I. 
56257-1 from Portugal). Emmcr: 


Khapli. Inheritance: Monogenic to 
polygenic. 

Mosaic, Marmor tritici. O. S. R.: 
Unknowm. P. S. R.: Butler, Chancel- 
lor, Royal, Thorne, Vigo. Inheritance: 
Unknown. 

FIBER GP..OPS: cotton, fiber flax. 
Cotton 

Wilt, Verticillium alho~atrum. O. S. 

R . : Gossypium barhauense^ G. hopii (Lew- 
ton), and G. hirsuium. P. S. R.: G. 
barbadense^ Tanguls from Peru, 1936, 
and American-Egyptian varieties; G. 
hopiiy Moencopi, and Sacaton aborig- 
inal; and Acala x Hopi x Acala crosses 
1-9 and 6-1-26; G. hirsutum, Delfos 
4-19, Acala 10-13, Acala 23-21, 
Acala 29-16, and Acala 4-42. Inherit- 
ance: Polygenic. 

Damping -OFF, Rkizoctonia solani. O. 

S. R.; Acala 29-16. P. S. R.: Now 
available as Acala 29-16, line 64. 
Inheritance: Unknow^n. 

Root knot, Meloidogyne sp. O. S. R.: 
Hopi, Moencopi, and Sacaton aborig- 
inal strairus, particularly the latter. 
P. S. R.: Acala x Hopi x Acala 1-9-56. 

I iihcri ta ncc : Pol ygenic . 

Fiber flax 

Rust, Afclampsora Uni. O. S. R.: 
Variety Ottawa 770B. P. S. R.: Cas- 
cade, released in 1945, and Tallmune.. 
Inheritance: Resistance is a mono- 
genic dominant. 

Wilt, Fusarium Uni. O. S. R.: 
Variety Ottawa 770B. P. S. R.: Va- 
rieties Cascade and Tallmune. Inher- 
itance: Multi f)le factor. 

Aster ocYSi’is, Aster oryxtis radicis. O. 
S. R.: Variety Hercules, an introduc- 
tion from Lincoln, New Zealand. 

FORAGE CROPS: alfalfa, clover, 
cowpeas, southern legumes, soybeans. 

Alfalfa 

Wilt, Corynebacterium insidiosum. O. 
S. R.: United States Department of 
Agriculture collections from Turkistan 



SOME SOURCES OF RESI 

in 1898 (Turkistan) and north India in 
1910 (Ladak). P. S. R.: Commercial 
varieties Ranger and Buffalo. Inherit- 
ance: Resistance is partially domi- 
nant, polygenic. 

Black stem, Ascochyta imperfecta, O. 
S. R.: Selected from commercial seed. 
P. S, R.: Clones in several breeding 
nurseries in Kansas. Inheritance: Sev- 
eral factors. 

Crown rot, Fusarium sp. O. S. R.: 
Plant differences within strains. P. S. 
R.: Same as preceding. Inheritance: 
Unknown. 

Crown and stem rot, Sclerotinia 
trifoliorum, O. S. R.: No clear strain or 
varietal difference. P. S. R.: Same as 
preceding. Inheritance: Unknown. 

Downy mh^dew, Pernnospora tri/o- 
liorvm. O. S. R.: Plant and strain 
differences. P. S. R.: Same as preced- 
ing. Inheritance: Unknown. 

Leaf spoi’, Psevdopeziza medicaginis, 

O. S. R.: Regionally adapted varieties. 

P. S. R.: Varieties Williamsburg, 
Narraganscit, and Atlantic. Inherit- 
ance: Resistance is dominant. 

Root rot, Rhizoctonia sp. O. S. R.: 
Plant and varietal differences. P. S. R.: 
Same as preceding. Inheritance is un- 
known. 

Rust, Urorryces striatus. O. S. R.: 
Mixed commercial seed stocks and 
varieties (Ladak). P. S. R.: Clones in 
several breeding nurseries in Nebraska. 
Inheritance: Multiple factors. 

Stem blioht and crown rot, 
Colletolrichum sp. O. S. R.: Strain and 
varietal differences. P. S. R.: Same as 
preceding. Inheritance: Unknown. 

Yellow leaf blotch, Pseudopeziza 
jonesii. O. S. R.: Plant differences exist. 
P. S. R.: None. 

Dwarf. O. S. R.: Plant differences 
in commercial varieties. P. S. R.: 
California Common No. 49. Inherit- 
ance: Unknown. 

Yellow virus. O. S. R.: Plant dif- 
ferences exist. P. S. R.; Same as 
preceding. Inheritance: Unknown. 

Pea aphid. O. S. R.: Selections. 
P. S. R.: Selected inbred and hybrid 
plants are being tested. None available 
commercially. Inheritance: Unknown. 


STANCE IN CROP PLANTS 197 

Clover 

Several species of Trifolium arc 
resistant to crown rot, Sclerotinia tri- 
foliorum^ and powdery mildew, Ery^ 
siphe polygon^ as reported in “Sclcro- 
tinia trifoliorum, a Pathogen of Ladino 
Clover” by K. W. Kreitlow, Phy^ 
topathology, volume 39, pages 158-166, 
*949» “Susceptibility of Some 
Species of Trifolium^ and Melilotus to 
Erysiphe polygon^ by K. W. Kreitlow, 
Plant Disease Reporter^ volume 32, 
pages 292-294, respectively. 

Large hop glover 

Powdery mildew, Erysiphe poljh 
goni, O. S. R.: Individual plant from a 
farm seed lot. P. S. R.: Resistant line 
in breeding material, North Florida 
Agricultural Experiment Station. In- 
heritance: Unknown. 

Red clover 

Crown rot, Sclerotinia trifoliorum, 
O, S. R.: Slight resistance in some 
farm strains grown where the organ- 
ism is prevalent. P. S. R.: Slight 
resistance in Kenland, Kentucky Agri- 
cultural Experiment Station; in Stev- 
ens, Maryland Agricultural Experi- 
ment Station; and in Pennscott, 
Pennsylvania Agricultural Experiment 
Station. Abridged list, Sanford, Vir- 
ginia Agricultural Experiment Sta- 
tion. Inheritance: Unknown. 

Northern anthracnose, Kabatiella 
caulivora, O. S. R.: Slight resistance in 
some farm strains grown where the 
organism is prevalent. P. S. R.: Highly 
resistant lines in breeding material, 
Wisconsin Agricultural Experiment 
Station; moderate resistance in Dol- 
lard, McDonald College, Quebec, 
Canada; some resistance in variety 
Ottawa, Dominion Experiment Farm, 
Ontario, Canada; abridged list, Pur- 
due, Indiana Agricultural Experiment 
Station; and Midland, composite of 
selected fann strains. Inheritance: 
Unknown. 

Powdery mildew, Erysiphe polygoni, 
O. S. R.: Occasional plants of farm 



YEARBOOK OF AORICULTURE 1953 


198 

strains have moderate resistance. 
P. S. R.: High resistance in Wisconsin 
Mildew Resistant, Wisconsin Agricul- 
tural Experiment Station. Inheritance: 
Unknown. 

Snow mold (unidentified low-tem- 
perature basidiomycete). O. S. R.: 
Siberian red clover (source not known). 
P. S. R.: Siberian Red, Alberta, 
Canada. Inheritance: Unknown. 

Southern anthracnose, Colletotri- 
chum Irifolii. O. S. R.: Some resistance 
in most of the farm strains grown where 
the organism is prevalent. P. S. R.: 
High resistance in Kenland, Kentucky 
Agricultural Experiment Station; in 
Tennessee Purple Seeded, Tennessee 
Agricultural Experiment Station. 
Some resistance in Tennessee Anthrac- 
nose Resistant, Tennessee Agricultural 
Experiment Station; in Kentucky 215, 
Kentucky. Abridged list, Sanford, 
Virginia; Pennscott, Pennsylvania; 
Stevens, Maryland; and Cumberland, 
composite of selected strains. Inherit- 
ance: Unknown. 

Sub clover 

Powdery mildew, Erysiphe polygonu 
O. S. R.: Unknown. P, S. R.: Seed 
originating in Australia. Inheritance: 
Unknown. 

White clover 

Crown rot, Sclerotinia Uijolmum, 

O. S. R.; Some resistance in some 
common white and Ladino stocks 
grown where the organism is prevalent. 

P. S. R.: Moderate resistance in lines 
of Ladino, United States Regional 
Pasture Research Laboratory, Penn- 
sylvania. Inheritance: Unknown. 

White sweetclover 

Black stem, Mycosphaerella Uthnlis, 
O. S. R.: Some resistance in plant 
introductions from lurkey. P. S. R.: 
Moderate resistance in lines of breed- 
ing material, Wisconsin Agricultural 
Experiment Station. Inheritance: Un- 
known. 


Crown rot, Sclerotinia trifoliofum, 
O. S. R.: Slight resistance in some seed 
lots that arc grown where the organ- 
ism is prevalent. P. S. R.: Willamette, 
Oregon Agricultural Experiment Sta- 
tion. Inheritance: Unknown. 

Gooseneck or stem canker, Ascoch- 
yta caulicola. O. S. R.: Some resistance 
in seed lots. P. S. R.: High resistance 
in lines of breeding material, Wiscon- 
sin Agricultural Experiment Station. 
Inheritance: Polygenic. 

Leaf spot, Cercospora davisii. O. S. 

R. : Some resistance in seed lot from 
Turkey. P. S. R.; Some resistance in 
lines of breeding material, Wisconsin 
Agricultural Experiment Station. In- 
heritance: Unknown. 

Root rot, Phytophthora cactorum. O. 

S. R.: Some resistance in some com- 
mon seed stocks grown where the or- 
ganism is prevalent. P. S. R.: Highly 
resistant lines in breeding material, 
Wisconsin Agricultural Experiment 
Station and Illinois Agricultural Ex- 
periment Station. Inheritance: Prob- 
ably polygenic dominant. 

Southern anthracnose, Colletotri^ 
chum tri/olii, O. S. R.: Some resistance 
in domestic seed lots. P. S. R.: High 
resistance in N-i, Nebraska Agricul- 
tural Experiment Station and in lines 
of breeding material, W'isconsin Agri- 
cultural Experiment Station. Inheri- 
tance: Unknown. 

COWPEAS 

Bacterial canker, Xanlhomonas wg- 
nicola. O. S. R.: Brabham (Iron x 
Whippoorwill), Buff, Iron, Six-Weeks- 
Ala., Suwannee, and Victor (Groit x 
Brabham); V. sinensis^ P. E. I. Nos. 
1 52 1 99 from Paraguay, 167284 from 
I’urkey, and 186456 from Nigeria; 
Vigna spp. P. E. I. Nos. 158831 from 
Paraguay, 171985 from Dominican 
Republic, 182025 from Liberia, 124606 
from India, 181584 from Union of 
South Africa, and 1592 10 from Union 
of South Africa. P. S. R.: Brabham, 
Buff, Iron, Suwannee, and Victor; P. 
E. I. Nos. 152199 from Paraguay, 
167284 from Turkey, and 186456 from 



SOME SOURCES OF RESISTANCE IN CROP PLANTS 


Nigeria; and selections from Chinese 
Red X Iron and Blackcyc x Iron hy- 
brids available from the Division of 
Plant Exploration and Introduction^ 
United States Department of Agricul- 
ture. Inheritance: Resistance is a 
monogenic dominant. 

Fusarium wilt, Fusarium oxysporum f. 
tracheiphilum, O. S. R.: Iron, Clay, and 
Virginia Blackeye. P. S.R.: Iron, Clay, 
Calhoun Crowder (from Clay x Large 
Speckled Crowder), and Calva Black- 
eye (from California x Virginia Black- 
eye). Inheritance: Resistance is a mono- 
genic dominant. 

Powdery mildew, Erysiphe polygoni. 
O. S. R.: Vigna sesquipedalis (Aspara- 
gus bean, Yardlong bean). P. S. R.: 
Selection from Yardlong x ‘‘Azul 
Grande” (New Era x Sugar Crowder 
selection) developed at Turrialba, 
Costa Rica. Inheritance: Multiple fac- 
tor with resistance recessive. There are 
conflicting reports in the literature on 
mode of inheritance, which suggests 
the possibility of distinct races of the 
pathogen. 

Root knot, Meloidogyne sp. O. S. 
R.: Iron, Clay, and Crow’der. P. S. R.: 
Iron, Clay, Crowder (from Clay x 
Large Speckled Crowder), and Calva 
Blackeye (from Calif, x Virginia Black- 
cye). Inherit;;;. nee: Unknown. 

Southern legi-mes 

Blue lupine {Lupinus angustifolium) 

Anthracnose, Glomerella cingulata. 

O. S. R.: P. E. I. Nos. 167938, 167943, 
168529, 168535 from Portugal. P. S. 
R.: Commercial types that are being 
developed. Inheritance: Unknown. 

LesPEDEZA STIPULACEA 

Powdery mildew, Microsphaera dif- 
fusa, O. S. R.: Old fields of lespedeza. 

P. S. R.: Commercial variety Rowan 
is being released. Inheritance: Condi- 
tioned by at least two genes. One or 
more of these genes are linked with 
certain genes associated with flower 
color. 


199 

Winter field pea {Pisum aruense) 

Root rot, Aphammyces euteiches. O. 
S. R.: An imported pea from Puerto 
Rico. P. S. R.: Romack, a resistant 
variety available in limited quantities. 
Inheritance: Unknown. 

Soybeans 

Bacterial blight. Pseudomonas glyc- 
inea. O. S. R.: High resistance in P. E. 
I. 68521 from Manchuria, P. E, I. 
68554-1 from Manchuria, and P. E. 
I. 1 532 13 from Belgium; moderate 
resistance in Hawkeye. P. S. R.: High 
resistance in P. E. I. 68521 from Man- 
churia, P. E. I. 1 532 1 3 from Belgium, 
P. E. I. 68554-1 from Manchuria, and 
N48-4860 (Haberlandt x Ogden); 
moderate resistance in Hawkeye. In- 
heritance: Multiple factor. 

Bacterial pustule, Xanihomonas 
phaseoli var. sojensis. O. S. R.: CNS 
(selection from Clemson, P. E. I. 
71659 from China), FC 31592. P. 
S. R.: Lines from hybrid populations 
with CNS as one parent: N46-2566, 
N47-309, N48-1574, N49-2560, D49- 
772, 1)49-2524, D49-2477, si-199, 
L9-409'. L 9 - 4 » 96 > 1 - 9 - 4 ' 97 - Inherit- 
ance: Resistance is a monogenic 
recessive. 

Wildfire, Pseudomonas labaci, O. S. 
R.: CNS (selection from Clemson, 
P. E. I. 71659 from China) and FC 
31592. P, S. R.: Lines from hybrid 
populations with CNS as one parent: 
N46-2566, N47-30Q, N48-1574, N49- 
2560, D49-772, 649-2524, D49-2477, 
Si-i 99 ) ^9-4091, L9-4196, L9-4197. 
Inheritance: Field resistance condi- 
tioned by resistance to bacterial pustule. 

DotVNY MILDEW', PcTonospora mflJi- 
shurica (3 physiologic races known). 
O. S R,: Races i and 3 aod moderate 
resistance to race 2: Chief, Dunfield, 
Manchu 3, Mukden, T 117; races un- 
known: Acadian and Ogden. P. S. R.: 
Races i and 3 and moderate resistance 
to race 2: Chief, Dunfield, Manchu 3, 
Mukden, T 117; races unknowm: 
Acadian and Ogden. Inheritance: 
Resistance to each of races 1,2, and 3 
is monogenic dominant. Resistance to 



200 


YiARBOOK OF AGRICULTURC 19S3 


race 3 of Richland is conditioned by 
two genes. 

Froceye, Cercospora sojina, O. S. R.: 
Adams, Lincoln, Anderson, Wabash, 
Roanoke, and FC 31592. P. S. R.: 
Adams, Lincoln, Anderson, Wabash, 
Roanoke, FC 31592, D49-772, D49- 
1633; Lincoln x (Lincoln x Richland), 
selections A6K-101 1, A6K-1801, A7- 
6102, A7-6103, A7-6402, A7-6520, 
C 739> C 745* ^ 764, H 6150, L6-1 152, 
L6-1503, L5-1656, L6-2132, L6- 
8179; Lincoln x (Richland x Early- 
ana), selections C 981 , C 976; Early ana 
x (Lincoln x Richland), selections 
C 996, C 997; Lincoln x Ogden selec- 
tion C 985; Lincoln x (Lincoln x 
C 1 71) selections L8-10755 and L8- 
10780. Inheritance: Resistance is a 
monogenic dominant. 

Purple seed stain, Cercospora kiku^ 
chii. O. S. R,: CNS. P. S. R.: N46-2566 
(S I oox CNS), N49”256o (S i 00 x CNS), 
CNS. Inheritance: Undetermined. 

Stem canker, Diaporthe phaseolorum 
var. hatatatis. O, S. R.: Unknown. 
P. S. R.: Partially resistant: 6K--1521, 
8T~8 i 2, 8T-1522, 87-1605. Inheri- 
tance: Unknown. 

Target spot, Corynespora cassiicola. 

O. S. R.: Ogden, Palmetto, Tarheel 
Black. P. S. R.: Ogden, Palmetto, 
Tarheel Black, N47“3479, D49-772, 
and D49-2573, Inheritance: Unde- 
termined. 

Root knot, MeLoidogyne spp. (soy- 
beans attacked by five species). O. S. 
R.: Resistant to some species of nema- 
todes: Palmetto, Si 00, and Laredo. 

P. S. R.: Resistant to some spiccies of 
nematodes: Palmetto, Si 00, N45-3799 
(Palmetto x Ogden), N46-2566 (Si 00 
X CNS), N46--2652 (Volstate x Pal- 
metto), and Laredo. Inheritance: 
Undetermined. 

FORESTRY : American chestnut, 
American elm, European field elm, 
mimosa, white pine. 

American chestnut 

, Chestnut blight, Endothia para- 
sitica, O. S. R.: Asiatic chestnut trees, 


particularly the Chinese chestnut and 
the Japanese chestnut. P. S. R.: Out- 
standing strains are being selected 
from introductions by the United 
States Department of Agriculture 
from the Orient made between 1927 
and 1932. Resistant hybrids between 
the Asiatic chestnuts and the native 
American chestnuts arc being de- 
veloped. Chinese chestnut trees, for 
nut production and ornamental use, 
are for sale by commercial nursery- 
men. Inheritance: Unknown. 

American elm 

Phloem necrosis (virus). O. S. R.: 
Field selections of American elm trees 
growing in central Kentucky. P. S. R.: 
Approximately one- half of the seed- 
lings produced by the selected trees are 
resistant to the disease. Inheritance: 
Unknown. 

European field elm 

Dutch elm disease, Ceratostomella 
ulmi, O. S. R.: European field elm, 
Ulmus carpinifolia^ and variety, Chris- 
tine Buisman, selected by Dutch 
pathologists. Imported from England 
in 1939 and released by the United 
.States Department of Agriculture to 
nui-serymcn. P. S. R.: Christine Buis- 
man. Ulmus pumila, the Siberian elm, 
is generally resistant. Inheritance: 
Unknown. 

Mimosa 

Mimosa w^ilt, Fusarium oxysporum f. 
perniciosum, O. S. R.: Seedling selec- 
tions of Albizzia julibrissin, P. S. R.: 
Two clones released to nurserymen by 
the United States Department of 
Agriculture for propagation in 1951 
as Tryon and U. S. No. 64. Inherit- 
ance: Unknown. 

White pine 

White pine blister rust, Cronar- 
tium fibicola. O. S. R.: Resistant strains 
of the white pine. P. S. R.: Resistant 
selections of white pine have been 
obtained and are being propagated 



SOME SOUECES OF tBSlSTANCE IN CEOF PLANTS 


201 


vegetatively by rooted cuttings. In- 
heritance: Unknown. 

FRUITS: Apple, apricot, black- 
berry, blueberry, cranberry, grapes, 
muscadine grapes, peach, pear, rasp- 
berry, strawberry. 

Apple 

Fire blight, Erwina amylovora, O. S. 
R.: Immunity has not been satisfac- 
torily demonstrated. Somewhat resist- 
ant commercial varieties include Deli- 
cious, Arkansas Black, and Winesap. 
Inheritance: Polygenic; resistance par- 
tially dominant. 

Apple scab, Venturia inaequalis. O. S. 
R.: Malus atrosanguinea (804), M. 
Aoribunda (821), M. micromalus (245- 

? ;8), M. prunifolia (19651), M, pumila 
R No. 12740-7A), Af. zumi calocarpa, 
Antonovka, and others. P. S. R.: Ca- 
thay, Elk River, Kola, Red Tip, S. D. 
Jonsib, Tipi, Zapata, and commercial 
types now being developed. Inherit- 
ance: Monogenic dominant in M, 
floribunda (821); two dominant genes 
in M. micromalus (245-38); three domi- 
nant genes in M, pumila (R No. 12740- 
7A); probably one major dominant 
gene in M. atrosanguinea (804), M, 
prunifolia (19651), and M. zumi calo- 
carpa. Polygenic in Antonovka. All 
clones listed are heterozygous for the 
resistant genes as listed. 

Cedar-apple rust, Gymnosporan- 
gium juniper i-virgimanae, O. S. R.: 
Arkansas Black, Delicious, McIntosh, 
Macoun, Winesap, and Wolf River. 
P. S. R.: Same as preceding. Inherit- 
ance: Monogenic dominant. Arkansas 
Black and McIntosh are homozygous, 
whereas others listed are heterozygous 
resistant. 

Apricot 

Brown rot, Monilinia laxa, O. S. R.: 
Tilton, Wenatchee Moorpark, Hems- 
kirke, and Hersey Moorpark arc mod- 
erately resistant. Moorpark and Peach 
are most resistant. P. S. R.: Preceding 
varieties and related seedlings. Inher- 
itance: Unknown. 


Blackberry 

Double blossom, Cercosporella rubi. 
O. S. R.: Varieties Himalaya and 
Rogers. P. S. R.: Varieties Brainerd 
and Himalaya. Inheritance: Un- 
known. 

Leaf spot, Mycosphaerella rubi, O. S. 
R.: Varieties Evergreen and Hima- 
laya. P. S. R.: Preceding varieties and 
also selections from breeding work 
of the United States Department of 
Agriculture and the Oregon Agricul- 
tural Experiment Station. Inheritance: 
Multiple factor with resistance par- 
tially dominant. 

Orange rust, Gymnoconia inter sti- 
tialis, O. S. R.: Varieties Evergreen, 
Eldorado, Snyder, and Lucretia. P. S. 
R.: Preceding varieties, as well as 
Young and Boysen. Inheritance: Un- 
known. 

Vertictllium wilt, Verticilliurn albo- 
atrum. O. S. R.: The varieties Ever- 
green and Himalaya and clones of 
Ruhus ursinus. P. S. R.: Varieties Ever- 
green, Himalaya, Logan, Mammoth, 
Cory, Thornless, and Burbank Thorn- 
less. Inheritance: Unknown. 

Blueberry 

Stem canker, Physalospora corticis. 

O. S. R.: Crabbe 6 selected from W'ild 
type in North Carolina and selections 
from commercial varieties. P. S. R.: 
Varieties Wolcott, Murphy, Angola, 
Crabbe 6, Adams, Scammcll, Jersey, 
Rubel, Harding, and all rabbiteye 
varieties selected from wild. Inheri- 
tance: Resistance probably dominant. 

Stunt virus. O. S. R.: Unknown. 

P. S. R.: Rancocas, in commercial use. 
Inheritance : I ■ nknown . 

Cranberry 

False blossom, Chlorogenus vacciniL 
O. S. R.: Various selections from wild 
cranberries in the eastern United 
States, including the commercial va- 
rieties McFarlin, Early Black, and 
Shaw’.s Success. P. S. R.: The preced- 
ing varieties and the recently intro- 
duced Wilcox variety. Hybrid selec- 



202 


YEARBOOK OF AGRICULTURE 1953 


tions are now under test by the United 
States Department of Agriculture and 
cooperating agencies. Inheritance; Re- 
sistance is actually klendusity, or es- 
cape, because the insect vectors do not 
feed on the plants. Klendusity is con- 
trolled by multiple factors. 

Grapes 

Downy mii.dew, Plasmopara vilicola. 

O. S. R.: Black Monukka; Jaeger 70; 
certain selections of Vitis rupestris^ such 
as Rupestris Martin and Rupestris 
Mission; V. liticecumii; several selec- 
tions of V. cinerea Nos. 23, 24, 27, 45, 
47, 48, and 54; V. cordijolia Nos. 15 
and 29; and V. riparia Nos. 13 and 50. 

P. S. R.: S. V. 12-375, 12-303, 12- 
309, 12-401, 23-18, 23-657, S. 6>68, 
5813, 14664, 15062, and 12 named 
varieties are reported highly resistant. 
Inheritance: Multiple factor. 

Powdery mildew, Uncinula necator, 
O. S. R.: Selections Nos. 23, 24, 27, 
45, 47, 53 of Vitis cinerea; No. 15 of 
V. cordijolia; and varieties Rupestris 
Martin and Rupestris Le Reux of F. 
rupestris, P. S. R. ; S 6468, 14664, 9110, 
11803, 15062, S. V. 12-303, 5-276, 
12-375, 23-18, 23-410. Inheritance: 
Probably multiple factor. 

Anthracnose, Sphaceloma nmpelinum, 

O. S. R.; Selections of Vitis cinerea Nos. 
23, 24, 27, 45, 47, and 54; F. cordijolia 
No. 15; and F. riparia Nos, 13 and 50. 

P. S. R.: S. 5455, S. V. 12-413, and 
23-501. Inheritance: Resistance reces- 
sive, multiple factor. 

Black rot, Guignardia bidwellii. O, 
S. R.: Selections from several wild 
species of F/ 7 iJ, principally F. cinerea^ 
V. cordijolia, and F. rupestris; selections 
of F. cinerea Nos. 23, 24, 27, 45, 47, 48, 
and 54; and F. cordijolia Nos. 15 and 
29 have thus far been free of the black 
rot. Rupestris Martin, a rootstock va- 
riety, and Seibel j,ooo, a French hy- 
brid wine type, have, shown no infec- 
tion. In the literature Gonderc Nos. 
28-112, 175 -38, 3304, and 162-97 are 
reported as immune, and 30 varieties 
arc reported as highly resistant. P. S. 
R.: Preceding varieties and commer- 


cial types now being developed. In- 
heritance: Unpublished data on sev- 
eral thousand vines artificially inocu- 
lated with the organism indicate that 
the resistance is apparently multiple 
factor with the very high resistance of 
F. cinerea strongly dominant in most 
crosses. 

Muscadine grape 

Black rot, Guignardia bidwellii f. 
muscadinii. O. S. R.: Breeding .selections 
of the United States Department of 
Agriculture. P. S. R.: Varieties Tar- 
heel and Topsail. Inheritance: Mul- 
tiple factor. 

Peach 

Bacterial spot, Xanthomonas pruni, 
O. S. R.: Unknown. P. S. R.: Varieties 
such as Hilcy Ranger and Belle of 
Georgia. Inheritance: Multiple factor. 

Powdery mildew, Sphaerotheca pan- 
nosa. O. S. R,: Unknown. P. S. R.: 
Eglandular type varieties. Inheritance: 
Monogenic. 

Root KNOT,^/r/v;<ygg>’Wf spp. O. S. R. : 
Mostly United States Department of 
Agriculture collections from India, 
China, and Turkestan. P. S. R.: Shalil, 
Yunnan, Bokhara, S-37, and some 
seedlings. Inheritance: Resistance is 
dominant; probably multiple factor. 

Peach mosaic, Marmor persicae. 

O. S. R. (Here we arc dealing with 
tolerance and not resistance). Many 
varieties of clingstone, such as Paloro, 
Peak, Phillips, and Sims, and a few 
varieties of freestone peaches, such as 
Erly-Red-Fre, Fisher, and Valiant, 
are highly tolerant. Most of the free- 
stones become severely damaged. 

P. S. R.: Commercial varieties, mostly 
of the clingstone type, are tolerant. 
Symptom development is complicated 
by many strains of the virus. Inherit- 
ance: Unknown. 

Pear 

Stony pit, virus. O. S. R.: Bartlett 
pears are symptomless carriers, but 
the virus remains in the tree. Of the 



SOME SOURCES OF RESISTANCE IN CROP PLANTS 


important varieties Bose is one of the 
most seriously affected. The Waite 
variety is susceptible, and since this 
is probably a cross bet\veen Bartlett 
and another variety, it appears the 
symptomless characters are not domi- 
nant. 

Fire blight, Envinia amylovora. 
P. S. R.: Immune, Richard Peters; 
highly resistant, Orient, Hood, and 
Pineapple; fairly resistant, Baldwin, 
Waite, and Ewart; slightly resistant, 
Kicffer. 

Raspberry 

Anthracnosb, Elsinoe veneta. O. S. 
R.: Ruhus coreanus^ R, bifianis, R. par- 
vi/oliuSj R. kuntzeanusy R. albescens. P. S. 
R.: Selections from breeding material 
of the North Carolina Agricultural 
Experiment Station. Inheritance: Mul- 
tiple factor. 

Late raspberry rust, Pucciniastrum 
amerkanum. O. S. R.; Rubus coreamtSy R. 
hifloruSy R. kunlzcanuSy R. mirifvlius, R. 
parvifoliuSy R. innominatus, R. lamheri- 
ianuSy and R, tephroides. P. S. R.: Selec- 
tions from breeding material of North 
Carolina Agricultural Experiment Sta- 
tion. Inheritance: Multiple hictor. 

Leaf spot, Sepwria rubi. O. S. R.: 
Rubus coreanuSy R. biJloruSy R. parvifolius., 
R. morifoliuSy R. wrightiiy R. alluscens, 
and R. innominatus. P. S. R.: The vari- 
eties Dixie and Van Fleet, and selec- 
tions from breeding material of the 
North Carolina Agricultural Experi- 
ment Station. Inheritance: Multiple 
factor. 

Raspberry mosaic escaping (resis- 
tant to aphid vector in United States). 
O. S. R.: Varieties Lloyd George, Her- 
bert, Newburgh, and Newman. P. S. 

R. : Many commercial varieties includ- 
ing Washington, Milton, and Septem- 
ber. Inheritance: Multiple factor with 
klendusity partially dominant. 

Strawberry 

Leaf scorch, Diplocarpon ear liana. O. 

S. R.: Fragaria virginiana. P. S. R.: 
Many commercial varieties, including 
Catskill, Midland, Fairfax, Howard 1 7, 


203 

Blakemore, and Southland. Inherit- 
ance: Unknown. 

I-EAF spot, Mycosphaerella Jragariae. 

O. S. R.: Fragaria chiloensis. P. S. R.: 
Many commercial varieties, including 
Fairfax, Massey, Midland, Southland, 
Howard 17, and Klonmore. Inherit- 
ance: Unknown. 

Red stele, Phytophthorafragariae (two 
raccJs). O. S. R.: The variety Aberdeen 
in the United States, a chance seedling 
that originated in New Jersey, and 
Scotland No. 52 of the West of Scot- 
land Agricultural Experiment Station. 

P. S. R.: The varieties Temple, Fair- 
land, Sparkle, Redcrop, Pathfinder, 
and Vermilion, and the Scottish vari- 
ety Climax. Selections from the breed- 
ing work of the United States Depart- 
ment of Agriculture and the Maryland 
and Oregon Agricultural Experiment 
Stations. Inheritance: Multiple factor 
with resistance partially dominant but 
complicated by several physiologic 
races. 

Verticillium wilt, Vertkillium al- 
bo-atrum. O. S. R.: Fragaria chiloensis. 
P. S. R.: The variety Sierra and selec- 
tions from the breeding \vork of the 
California Agricultural Experiment 
Station and the California Straw- 
berry Institute. Inheritance: Unknown. 

GRASSES: Bahiagrass, Berinuda- 
grass, smooth broine, mountain brome, 
orchardgrass, slender wheatgrass, Su- 
dangra.ss, tall fescue, meadow fescue, 
tall oatgrass, timothy, western wheat- 
grass, sand bluestem, side-oats grama 
blue grama, buffalograss. 

Bahiagrass {Paspalurn notatum) 

Hehnintfmporiurn sativum. O. S. R.; 
Ciol lection of United Stages Depart- 
ment of Agriculture, P. E. I. 148966, 
which was collected in Argentina. 
P. S. R.: Argentina Bahia. Inheritance: 
Unknown. 

Bermuda-grass {Cynodon daciylon) 

Helminthosporium cynodontis. O. S. R.: 
Collection of United States Depart- 
ment of Agriculture, which was col- 



YEARBOOK OF AGRICULTURE 1953 


204 

Iccted in South Africa, P. E. I. 105933 
and 105935. P. S. R.: Coastal ^r- 
muda. Inheritance; Probably multiple 
factor. 

Smooth brome (Bromus inermis) 

Brown spot, Pyrenophora hromi. O. S. 

R. : Plant selected from Nebraska 
39-3400. P. S. R.: Has not been in- 
creased. Inheritance: Unknown. 

Mountain brome {Bromus marginatus) 

Head smut, Ustilago bullata. O. S. R.: 
Seed collections at Pullman, Wash. 
P. S. R.: Brornar Mountain brome- 
grass. Inheritance: Unknown. 

Orchardgrass {Daclylis glomerata) 

Leaf streak, Scolecotrkhum graminis. 

O. S. R.: One plant selected in old 
field near Taney town, Md., and one 
plant from S. C. S. 7060. P. S. R.: 
Preceding selections have not been 
increased. Inheritance: Unknown. 

Slender wheatgrass {Agropjnon tra- 
chycaulum) 

Head smut, Ustilago bullata, O, S. R .: 
Canadian collection from Alberta. 

P. S. R.: Commercial variety Fyra. 
Inheritance: Unknown. 

Leaf rust, Puccinia rubigo-vera, O. 

S. R.: Forest Service collection near 
Beebe, Mont., in 1933. P. S. R.: 
Commercial variety Primar, Inherit- 
ance: Unknown. 

Stripe rust, Puccinia glumarum, O. 
S. R.: Single plant collection in 
Saskatchewan, Canada, in 1923. P, S. 
R.: Commercial variety Mecca. In- 
heritance: Unknown. 

Stem rust, Puccinia graminis. O. S. 
R.: Canadian collection in Alberta. 
P. S. R.: Commercial variety Fyra. 
Inheritance: Unknown. 

Sudangrass {Sorghum vulgar e var. 
sudanense) 

Pseudomonas andropogoni, O. S. R.: 
Leoti sorghum. P. S. R.: Tift Sudan. 


Inheritance: Probably multiple factor. 

Anthracnose {Colletotrichtm graminis 
cola). O. S. R.: Leoti and other varie- 
ties of sorghum of similar resistance. 
P. S. R.: Varieties Sweet, Piper, and 
Tift. Inheritance: Probably multiple 
factor. 

Gloeocercospora sorghi. O. S. R.: Leoti 
sorghum. P. S. R.: Tift Sudan. In- 
heritance: Probably multiple factor. 

Leaf blight, Helminthosporium turcu 
cum, O. S. R.: Leoti and other 
varieties of sorghum of similar resist- 
ance. P. S. R.: Varieties Sw^eet, Piper, 
and Tift. Inheritance: Probaldy mul- 
tiple factor. 

Tall fescue {Festuca arundinacea) 

Crown ri;st, Puccinia coronata. O. S. * 
R.: Plant selection in 4-year-old 
planting of tall fescue at Corvallis, 
Oreg. P. S. R.: Alta fescue. Inherit- 
ance: Unknown. 

Meadow fescue {Festuca elatior) 

Crown rust, Puccinia coronata, O. S. 
R.: Two plants selected from old field 
in Maine. P. S. R.: Selections have 
not been increased. Inheritance: Mul- 
tiple factors. 

Tali, oatgrass {Arrhenatherum elaiius) 

Leaf smut, Puccinia rubigo-vera. O. S. 

R. : Varieties Tualatin and S. C. S. 
nonshattcring. P. S. R.: Tualatin and 

S. C. S. nonshattering have not l^een 
increased. Inheritance: Unknown. 

Timothy {Phleum praiense) 

Stem rust, Puccinia graminis. O. S. 
R.: Minnesota 79 and 81, Svalov 523, 
Cornell 1676, and F. C. 12468 Ohio. 
P. S. R.: Varieties Milton and Man- 
etta. Inheritance: Monogenic domi- 
nant. 

Western wheatgrass {Agropyron 
smiihii) 

Rust, Puccinia rubigo-vera. O. S. R.: 
Some selected individuals appear to be 



SOME SOURCES OP RESISTANCE IN CROR PLANTS 


resistant. P. S. R.: Same as preceding. 
Inheritance: Unknown. 

Sand bluestem {Andropogon hall it) 

Rust, Paccinia sp. O. S. R. : Selected 
native strains. P. S. R. : Strain W2 con- 
tains many resistant plants in the pop- 
ulation. Inheritance: Unknown. 

SlDE-OATS GRAMA {BouUloUa 
curtipendula) 

Rust, Puccinia vexans. O. S. R.: In- 
dividual plants in sexual populations. 
Most of the apomictic strains have 
some resistance also. P. S. R.: T\icson 
variety released; Hope and Encinofjo, 
not released. Inheritance: Probably 
one or two factors in sexual types. 
Inheritance in apoinicts unknown. 

Blue grama (Bovielova gracilis) 

Rust, Puccinia vexans. O. S. R.: 
Individual plants in segregating popu- 
lations. P. S. R.: Commercial types 
arc being developed. Inheritance: 
Pro]>ably multiple factor. 

Buffalograss {Buchloe dactyloidcs) 

Leaf spot, Helminlhosporium incon^ 
spicuum. O. S. R.: Individual plants in 
selected populations. P. S. R,: In 
breeding stock. Inheritance: Unknown. 

Rust, Puccinia kansensis. O, S. R.: 
Individual plants in .segregating popu- 
lations. P. S. R.: In breeding stock. 
Inheritance: Unknown. 

HOPS 

Downy mildew, Pseudoperonuspora 
humuli. O. S. R.: Chance seedling 
obtained in flower garden, Hors- 
monden, Kent, England, in 1861. 
P. S. R.: Fuggles and a number of 
related seedlings. Inheritance: Un- 
determined. 

NUTS: Chinese chestnut, filbert, 
pecan, Persian (English) walnut, east- 
ern black walnut. 


205 

Chinese chestnut 

Chestnut blight, Endoihia parasitica. 
O. S. R.: Most varieties and seedlings 
arc highly resistant. P. S. R.: Varieties 
and seedlings. 

Phytophthora root disease, Phyto^ 
phthora cinnamomi. O. S. R.: Most 
varieties and seedlings are highly re- 
sistant. P. S. R.: Most varieties and 
seedlings. 

Twig canker, Cryplodiaporthe cash 
anea. O. S. R.: All varieties and seed- 
lings are resistant when grown on 
proper sites. P. S. R.: All varieties 
and seedlings. 

Twig canker, Botryospkaeria ribis 
chromogena. O. S. R.: All varieties and 
seedlings are resistant ^^•hen grown on 
proper sites. P. S. R.: All varieties and 
seedlings. 

Filbert 

Filbert bacterial blight, Xan ^ 
thomonas corylina. O. S. R.: No immu- 
nity. Da\uana and Bolwyller are most 
resistant in ihc Pacific Northwest. 
'Fhese came from seedlings from a 
mixed population. P. S. R.: Preceding 
varieties. Inheritance: Unknown. 

Labreli.a leaf spot, Labrella coryli. 
O. S. R.: Variety Potomac shows some 
resistance. P. S. R.: Potomac. 

Pecan 

Scab, Cladosporiurn cjfusim. O. S. R.: 
\^arictics Stuart and Curtis are highly 
resistant. P. S. R.: Stuart and Curtis 
varieties. 

Bunch disease, or VVitches'-broom, 
virus. O. S. R.: Great diflerencc in 
varieties; Schley and Mahan most 
susceptible. Stuart resistant or symp- 
tomless carrier. P. S. R.: Stuart. 

Downy spot, Mycosphaerclla cary^ 
igenn. O. S. R.: Schley and a few other 
varieties are highly resistant. P. S. R.: 
Scliley and a few other varieties. 

Persian (English) walnut 

Walnut bacterial blight, Xan- 
thornonas juglandis. O. S. R.: Immunity 



YEARBOOK OF AGRICULTURE 1953 


206 

not Icnown. Eureka, San Jose, and 
Ehrhardt show some degree of resist- 
ance. Some seedling trees are seldom 
badly infected. In Oregon the Pari- 
sienne variety is somewhat resistant. 
Resistant varieties came originally 
from seedlings from a mixed popula- 
tion. P. S. R.: Eureka, San Jose, and 
Ehrhardt. Inheritance: Unknown. 
Branch wilt, Hendersonula toruloidea. 

O. S. R.: Meylan, Eureka, Blackmer, 
Payne, and Concord are somewhat 
resistant. Concord is the most resistant 
of all. Varieties originally came from 
seedlings from a mixed population. 

P. S. R.: Preceding varieties. Inherit- 
ance: Unknown. 

Crown rot, Phytophthcra cactorum. 

O. S. R.: All Persian varieties are 
susceptible. Paradox hybrids (Persian 
X J. hindsii) show some resistance. 

P. S. R.: Commercial Paradox hybrids. 
Root lesicjn, nematode injury, Para- 

tylenchtis vulnus and Cacopaurus pestis. 
t). S. R.: All Persian varieties are 
susceptible; Paradox hybrids (Persian 
X J. hindsii) show some resistance. 
P. S. R.: Commercial Paradox hybrids. 
Inheritance: Unknowm. 

Easter.n black walnut 

Anthracnose, Marssonina juglandis. 
O. S. R.; Varieties Ohio and Thomas 
arc somewhat resistant. P. S, R.: Ohio 
and Thomas. Inheritance: Unknown. 

OIL PLANTS: grain flax, safflower, 
peppermint, spearmint. 

Grain flax 

Rust, Melampsora Uni. O. S. R.: 
Varieties Ottawa 770B, Buda, J. W. 
S„ Pale Blue Crimped, Kenya, Wil- 
liston Golden, Morye, Rio, Minn. 25- 
107, Newland, Bolley Golden, Billings, 
Pale Verbena, Victory A, Bombay, 
Akmolinsk, Abyssinian, Leona, and 
Tammes’ Pale Blue. P. S. R.: Original 
and selections of original. Inheritance: 
Resistance is a monogenic dominant. 

Wilt, Fasarium Uni. O. S. R.: Varie- 
ties North Dakota Resistant No. 114, 


Bombay, Morye, Redwing, Buda, Ot* 
tawa 770B, Bison, and Pinnacle. P. S. 

R. : Preceding varieties and selections 
and hybrids of these. Inheritance: 
Multiple factor. 

Safflowter 

Rust, Puccinia carthami. O. S. R.: 
Introductions by Nebraska Agricul- 
tural Experiment Station from Ru- 
mania, Turkey, India, Egypt, and 
France. Resistant lines have been 
purified by the Nebraska Agricultural 
Experiment Station; others are being 
developed by the United States De- 
partment of Agriculture. Inheritance: 
Resistance is monogenic dominant. 

Root rot, Phyiophthora dreschleri, O. . 

S. R.: Introductions by the Nebraska 
Agricultural Experiment Station from 
Egypt and other introductions pre- 
sumed to have originated in Russia. 
P, S. R.: V'^arious degrees of root rot 
resistance in commercial varieties N-3, 
N--4, N-6, and N~8, developed by 
Nebraska Agricultural Experiment 
Station. Root rot resistant varieties 
are in process of development by the 
United States Department of Agricul- 
ture. Inheritance; Unknown. 

Peppermint 

Verticillium wu.t, Verticil Hum albo- 
atrum var. menthae. O. S. R.: Mentha 
crispa from unknown European source. 
P. S. R.: Clonal line maintained at 
Michigan State in regional mint nur- 
sery, East Lansing, Mich. Commercial 
types are in process of development at 
Michigan State College, Purdue Uni- 
versity, United States Department of 
Agriculture, and A. M. Todd Co., 
Kalamazoo, Mich. Inheritance: Poly- 
genic; not completely understood. 

Spearmint 

Spearmint rust, Puccinia menthae. O. 
S. R.: Mentha crispa from unknown 
European source. P. S. R.: Clonal line 
maintained at Michigan State College 
in regional mint nursery, East Lan- 



SOME SOURCES OF RESISTANCE IN CROP PLANTS 


sing, Mich. Commercial types in proc- 
ess of development at Michigan State 
College, Purdue University, United 
States I^partment of Agriculture, and 

A. M. Todd Company, Kalamazoo, 
Mich. Inheritance: Polygenic but not 
completely understood. 

SNAPDRAGON 

Rust, Puccinia antirrhini. O. S. R.: 
Antirrhinum majus strains from Dr. E. 

B. Mains. P. S. R.: The varieties Ar- 
tistic, Campfire, Loveliness, Red Cross, 
Snow Giant, Yellow Giant, Rosalie, 
Alaska, Apple Blossom, Canary Bird, 
Copper King, Crimson, and other com- 
mercial types now being developed 
Inheritance: Monogenic dominant. 

SUGAR CROPS: sorgo, sugar beet, 
sugarcane. 

Sorgo 

Leaf anthracnose and stalk red 
ROT, Colletotrichum graminicola, O. S. R.: 
Collections made in Africa in 1 945 by 
the United States Department of Ag- 
riculture. P. S. R.: Sart and noncom- 
mercial types. Inheritance: Resistance 
is inherited as monogenic dominant. 

Sugar beet 

Black root, Aphanomyces cochlioides* 
O. S. R.: U. S. 216 and other U. S. 
varieties developed in leaf-spot resis- 
tance breeding project. P. S. R,: Seed 
increase of S. P. I. 48B3-00, now re- 
leased as U. S. 1177, and related vari- 
eties; also varieties developed by beet 
sugar industry. Inheritance: Resis- 
tance is dominant in Fi, resistant x 
susceptible. Evidently disease reaction 
is conditioned by more than one pair 
of genes. 

Cergospora leaf spot, Cercospora 
beticola, O. S. R.: Inbred lines estab- 
lished from European open-pollinated 
varieties. P. S. R.: U. S. 216, U. S. 225, 
U. S. 226, and hybrid combinations of 
these; also in varieties developed by 
the beet sugar industry. Inheritance: 
Disease reaction of the F|, resistant x 


207 

susceptible, is intermediate. Segrega- 
tion in F2 indicates that disease reac- 
tion is conditioned by more than a 
single pair of genes. 

Downy mildew, Peronospora schacktiL 
O. S. R.: U. S. 15 and other selections 
from commercial and U. S. sugar beet 
varieties. P. S. R.: Tolerant Commer- 
cial varieties and highly resistant in- 
breds. Inheritance: Unknown. 

Curly top. Ruga vmucosans, O. S. 
R.: Selection out of heterogeneous 
commercial beet population released 
as U. S. No. I in 1933. P. S. R.: U. S. 
22/3. Inheritance: Probably one major 
gene for resistance with modifiers. 

Sugarcane 

Red rot, Physalospora tucumanensis, 

O. S. R.: Some forms of Saccharum 
spontaneum and probably of S. barberi. 

P. S. R.: Commercial and unreleased 
clones that are interspecific hybrids 
with S, spontaneum or 5 . barberi inherit- 
ance, such as CO 281, CP 28/11, 
CP 36/105, and CP 44/101. Inherit- 
ance: Undetermined. 

Root rot, Pythium arrhenomanes, 

O. S. R.: Various forms of Saccharum 
spontaneum and S. sinense, P, S. R.: 
Some commercial and unreleased 
clones that are interspecific hybrids 
with inheritance from these species, 
such as GO 290, CP 28/ri, CP 807, 
CP 33/409, and Kassoer. Inheritance: 
Undetermined. 

Mosaic, M armor sacchari. O. S. R.: 
All known forms of Saccharum spon^ 
taneum except those from Turkestan. 

P. S. R.: Numerous commercial and 
unreleased clones that are interspe- 
cific hybrids, usually with S, span- 
taneum inheritance. Inheritance: Un- 
determined. 

TOBACCO 

Bacterial wilt, Pseudomonas solan- 
acearum. O. S. R.: T. I. 448A, a selec- 
tion out of a collection made in Colom- 
bia, South America, 1942. P. S. R.: 
Oxford 26 and Dixie Bright loi. 
Inheritance: Polygenic. 



2o8 


YEARBOOK OF AGRICULTURE 1953 


Wildfire, Pseudomonas tabaci. O. S- 
R. : Kicotiana longiflora immunity trans- 
ferred to tobacco, 1947. P. S. R.: 
Commercial wildfire immune varieties 
will be available soon. Inheritance: 
Monogenic dominant. 

Black root rot, Thielaviopsis hasi~ 
cola, O. S. R.: Isolated naturally 
occurring strains of Havana and Bur- 
ley types. P. S. R.: Havana 142, 
Burley i, and other commercial 
varieties. Inheritance; Polygenic. 

Black shank, Phytophthora parasitica 
var, nicotianae. O. S. R.: Florida 301, 
obtained by crossing and selection 
within Nicotiana iahacum in 1931. P. S. 
R.: RG, Oxford i, Vesta 33, Dixie 
Bright 1 01, and other commercial 
varieties. Inheritance: Polygenic. 

Tobacco mosaic. O. S. R.: Nico- 
tiana glutinosa immunity transferred to 
tobacco 1938. P. S. R.: Kentucky 56, 
Vamorr 50, and other commercial 
varieties. Inheritance: Monogenic 
dominant, 

VEGETABLES: asparagus, bean, 
celery, crucifers, cucumber, lettuce, 
lima bean, rnuskmelons, onion, pea, 
peanut, peppers, potatoes, spinach, 
sweetpotato, tomato, watermelon. 

Asparagus 

Asparagus rust, Puednia asparagi, 
O. S. R.: Martha Washington and 
Mary Washington developed Ijy J. B. 
Norton from a male plant of unknown 
origin named Washington and female 
plants named Martha and Mary, 
selected from Reading Giant. P. S. R.: 
Above-named varieties and No. 500, 
developed by the California Agricul- 
tural Experiment Station. Inheritance: 
Probably polygenic. 

Bean 

Hai.o bag'ierial blight. Pseudo- 
monas phaseolicola. O. S. R.: Most field 
bean varieties: Pinto, Great Northern, 
Michelitc, and Red Mexican. P. S. R.: 
Preceding varieties; Pinto, University 
of Idaho Nos. 72, 78, and 1 1 1 ; Great 
Northern, University of Idaho Nos. 16, 


31, and 123; Michelite and Red 
Mexican, University of Idaho Nos. 3 
and 34; and Fullgreen. Inheritance: 
One 01 two recessive factors depending 
on the resistant and susceptible parents 
used. 

Anthracnose, Colleiotrichum linde- 
muthianum. O. S. R.: Alpha race: Wells 
Red Kidney, Cranberry, and Emmer- 
son 847. Beta race: Michelite, Pinto, 
Perry Marrow, and Emmerson 847. 
Gamma race: Robust, Perry Marrow, 
and California Small White. P. S. R.: 
Red Kidney and the preceding varie- 
ties. No commercial varieties are 
resistant to the three races. Inherit- 
ance: Single dominant factor pair for 
each race. When two or three races 
arc involved resistance is governed by 
two or three dominant factor pair 
differences, respectively. 

Be.an rust, IJromyces phaseoli typica, 
O. S. R.: No strain is resistant to all 
physiologic races, A number are 
resistant to most of the races: No. 780, 
a White Kentucky Wonder type; No. 
765, a Kentucky Wonder Wax type; 
and No. 814, a brown-seeded Ken- 
tucky Wonder Wax type. P. S. R.: 
U. S. Pinto Nos. 5 and 14 and Golden 
Gate Wax. Inheritance: Single domi- 
nant factor pair for each race thus far 
investigated. 

Powdery mildew, Erysiphe polygoni, 
O. S. R.; No strain or variety is re- 
sistant to all races. Pinto, U. S. 5 
Refugee, and Ideal Market arc re- 
sistant to a number of races. P. S. R.: 
Available in the preceding varieties 
and in others, such as Toperop, Logan, 
and Contender. Inheritance: Single 
dominant factor pair. 

Common bean mosaic, Marnior pha^ 
seoli, O. S. R.: Corl)ctt Refugee and 
Great Northern No. i. P. S. R.: Many 
commercial varieties and Idaho Ref- 
ugee, U. S. 5 Refugee, Sensation 
Refugee Nos. 1066 and 1071, Rival, 
and Toperop. Inheritance: There are 
two types of inheritance depending on 
resistant variety used. Corbett Refu- 
gee type, single dominant factor; 
Great Northern or Robust type, single 
recessive factor. 



SOME SOURCES OP RESISTANCE IN CROP PLANTS 


Curly top, Ruga verrucosans. O. S. 
R,: Varieties Pioneer, California Pink, 
Burtner’s Blightless, and Red Mexi- 
can. P. S. R.; Red Mexican, Univer- 
sity of Idaho Nos. 3 and 34; Great 
Northern, University of Idaho Nos. 
16 and 31; and Pinto, University of 
Idaho Nos. 72, 78, and in. Inherit- 
ance: Resistance is probably con- 
trolled by two genes, one of which is 
dominant to its allele, the other reces- 
sive to its allele. In a progeny segre- 
gating for both, the gene that is dom- 
inant to its allele is epistatic to the gene 
that is recessive to its allele. 

New York 15 mosaic. O. S. R.: 
Great Northern No. i and No. 123. 
P. S. R.: Toperop, Rival, Idaho Refu- 
gee, Great Northern, University of 
Idaho Nos. 123 and 16. Inheritance: 
Unknown. 

Pod mottle virus, Marmor valvo- 
lorum, O. S. R.: All local lesion sus- 
ceptible varieties arc considered com- 
mercially resistant: Great Northern, 
Pinto, Toperop, Rival, U. S. 5 Refu- 
gee, and others. P. S. R.: Available in 
preceding varieties. Inheritance: Sin- 
gle factor with local lesion infection 
dominant. 

Southern bean mosaic, Marmor 
laesiofaciens. O, S. R. : All local lesion 
susceptible varieties arc considered 
commercially resistant; Pinto, Great 
Northern, Blue Lake, Ideal Market, 
Kentucky W onder, and others. P. S. 
R.: Available in preceding varieties. 
Inheritance: Single factor with local 
lesion infection dominant. 

Celery 

Early blight, Cercospora apii, 
O. S. R.: Danish celery received for 
trial by Eastern States Farmers’ Ex- 
change and P. E. I. 11 5557 and 120875 
obtained from Turkey. P. S. R.: Emer- 
son Pascal has moderate resistance 
and breeding lines not yet commercial 
have as high resistance as the original 
sources and improved. Inheritance; 
Multiple factor. 

Late blight, Septoria apii-graveolmtis, 
O. S. R.: Danish celery received for 


209 

trial by Eastern States Farmers’ Ex- 
change; P. E. I. 176869 from Turkey 
may have a little higher resistance; 
and P. E. I. 1 15557 and P. E. I. 120875 
from Turkey. P. S. R.: Emerson 
Pascal, Giant Pascal, and White 
Plume have moderate resistance. 
Breeding lines with high resistance are 
available but they arc far from com- 
mercial type. Inheritance: Multiple 
factor. 

Yellows, Fusarium apii and F. apii 
var. pallidum. O. S. R.: Occasional 
plants in self-blanching varieties and 
most plants in most green varieties. 
P. S. R. : Michigan Golden, Cornell 19, 
and numerous green varieties. Inherit- 
ance: Information not definite, but a 
single dominant gene is probably 
responsible for most of the resistance. 

Crucifers 

Cabbage yellows, Fusarium oxyspo-- 
rum f. conglutinans. O. S. R.: Commer- 
cial varieties of cabbage Wisconsin 
Ballhead and W'isconsin Hollander 
and selections from a susceptible vari- 
ety Danish Ballhead. P. S. R.: Preced- 
ing varieties and numerous commercial 
cabbage varieties now in use. Nine 
varieties resistant to type A have been 
released. Inheritance: Type A, found 
in Wisconsin Ballhead, is monogenic 
dominant; type B, found in Wisconsin 
Hollander, is polygenic and becomes 
unstable when the soil temperature is 
unusually high. 

Clubroot, Plasmodiophora hrassicae. 
O. S. R.: Commercial varieties of 
turnip. P. S. R.: Resistant varieties 
of stock turnip are Bruce, May, and 
Dale’s Hybrid; resistant varieties of 
nitabaga are W^ilhelmsburger, Re- 
sistant Baugholm, and Immuna 11 . 
Inheritance: Polygenic. 

Mosaic, virus. O. S. R.: Commercial 
varieties of cabbage. P. S. R.: Im- 
proved All Seasons cabbage. Inherit- 
ance: Resistance to the mottle phase is 
incompletely dominant polygenic, but 
controlled by relatively few genes. 
Resistance to the chlorosis symptom, 
incited by the B virus, also is incom- 



210 


YEARBOOK OF AORICULTURE 1953 


pletely dominant and appears to be 
inherited quantitatively. Resistance 
to mottling seems to be independent 
of resistance to the chlorosis symptom. 

Cucumber 

Bacterial wilt, Erwinia iracheiphila. 

O. S. R.: Tokyo Long Green (wilt 
tolerance only). P. S. R.: Tokyo Long 
GrCen. No variety has been intro- 
duced. Inheritance: Not determined. 

Downy mildew, Pseudoperonospora 
cubensis. O. S. R.: Chinese Long, which 
is used in South Carolina and Puerto 
Rico, and Bangalore, an Indian varie- 
ty that is used in Louisiana. P. S. R.: 
Puerto Rico Nos. 39 and 40, Palmetto, 
Santee, and Surecrop. Inheritance: 
Not definite. There is some segrega- 
tion for resistance in the F2 hut no 
definite ratio. Resistant plants arc 
tolerant hut show some infection late 
in season. They will produce a crop 
where susceptible varieties fail. 

Scab, Cladosporium cucumerimm. O. S. 
R. : Late-maturing slicing varieties 
Longfellow and Windermoor Wonder. 

P. S. R,: Maine No. 2 and an im- 
proved slicing variety Highmoor. A 
scab-resistant National Pickling type 
may be released soon. Inheritance: 
Monogenic dominant. Plants have 
very high resistance. 

Cucumber mos.mc, Marmor cucumeris. 

O, S. R.: Chinese Long and Tokyo 
Long Green have tolerance only. 

P. S. R.: Pickling varieties Ohio 31, 
Ohio MR- 1 7, Yorkstate Pickling and 
slicing varieties Niagara, Surecrop, 
Burpee Hybrid, Puerto Rico 10, and 
Puerto Rico 17. Inheritance: Shifriss 
et al. state that three complementary 
genes apparently control the appear- 
ance or nonappearance of mottling in 
the cotyledon stage, the genetic ratio 
in the F2 being 27 nonchlorotic to 37 
chlorotic. The ratio of 27 : 37 is con- 
stantly changing in the true leaf stage. 
They state: “At this point several gene 
modifiers also take part in the genetical 
control of virus symptoms. Thus, the 
frequency of symptomless plants is 
very low.” 


Lettuce 

Downy mildew, Bremia lactucae, 
O. S. R.: There arc several com- 
mercial varieties each resistant to 
single biotypes of the fungus but none 
resistant to all biotypes. P. S. R.: 
Some of the Imperial varieties. In- 
heritance: Resistance is a monogenic 
dominant. 

Powdery mildew, Erysiphe cichor* 
acearum, O. S. R.: Most cultivated 
varieties. P. S. R.: Bell May (Mass.), 
Imperial varieties, Great Lakes, etc. 
Inheritance: Resistance is a monogenic 
dominant. 

Brown blight, cause undetermined. 
O. S. R.: Big Boston, White Chavigne, 
individual plants wdthin variety, and 
New York. P. S. R.: Imperial varieties 
and Great Lakes. Inheritance: Un- 
known. 

7’ipburn (physiological breakdown). 

O. S. R.: Slobolt x Great Lakes seg- 
regate (U. S. D. A.). P. S. R.: Vari- 
eties are being developed. Inheritance: 
Unknow'n. 

Mosaic. O. S. R.: P. E. I. 120965. 

P. S. R.: Parris Island (cos type;. 
Inheritance: Unknowm. 

Lima bean 

Downy mildew, Pfiylophthora phaseoli. 
O. S. R.: P. E. I. 164155 from India 
and 163580 from Guatemala. P. S. R.: 
Same as preceding hut not available 
in any commercial variety as yet. In- 
heritance: Single dominant factor pair. 

Lima bean mosaic, Marmor cucu- 
meris var. phaseoli. O. S. R.: Fordhook 
types. P. S. R.: Concentrated Ford- 
hook, Fordhook 242, and Regular 
Fordhook. Inheritance: Two dominant 
complementary factors. 

Muskmelons 

Alternaria leaf blight, Alternaria 
cucumerina. O. S. R.: MWR 3915 
(Indiana). P. S. R.: Purdue 44. 
Inheritance: Unknown, 

Downy mildew, Pseudoperonospora 
cubensis. U. S. D. A. melon-breeding 
Accession No. 29554 and P. E. I. 



SOME SOURCES OF RESISTANCE IN CROP PLANTS 


2II 


1241 12 from India and Cuban Castil- 
lian. P. S. R.: Commercial types in 
process of development and Georgia 
47, Rio Sweet, and Weslaco H and F. 
Inheritance: Unknown. 

Fusarium wilt, Fusarium oxysporum, 

O. S. R.: Certain plants in Honey 
Dew. P. S. R.: Golden Gopher and 
Iroquois. Inheritance: Probably one or 
two dominant factors. 

Marssonina blight, Marssonina mel~ 
onis. O. S. R.: Freeman cucumber. 

P. S. R.: Lines are being developed at 
Cornell University. Inheritance: 
Unknown. 

PowqjERY MILDEW, Erysiphe cichora- 
cearum, O. S. R.: Varieties from India, 
California Accession No. 525, U. S. 
D. A. melon-breeding Accession No. 
29554, and P. E. I. 79376. P. S, R.: 
PMR Cantaloupe No. 45, resistant to 
Race I, and PMR Cantaloupes Nos. 
5 and 6, resistant to all known races, 
and Georgia 47. Inheritance: Probably 
dependent upon a dominant single 
factor with several modifiers. 

Mosaic, Marmor cucumeris, O. S. R.: 
Freeman cucumber (C. melo var. 
conoman). P. S. R.: Lines with mosaic 
resistance and improved type are be- 
ing developed at Cornell University. 
Inheritance: Unknown. 

Onion 

Black mold, Aspergillus niger. P. S. 

R. : White varieties, such as So\Uhport 
White Globe and White Portugal, are 
resistant. Inheritance: Resistance is 
perfectly correlated with dry scale 
color. Sec statement under smudge 
regarding inheritance of dry scale 
color. 

Downy mildew, Peronospora destructor. 

O. S. R.: Italian Red 13-53, self-sterile 
cross-fertile, propagated by top sets. 

P. S. R.: Preceding and Calred. Inher- 
itance: Resistance of scapes is condi- 
tioned by two duplicate recessive genes. 
There is no association between foliage 
and scape resistance. 

Pink root, Pyrenochaeta terrestris. O. 

S. R.: Allium fistulosum and individual 
plants in Crystal Wax and Yellow 


Bermuda. P. S. R.: Preceding vari- 
eties and Yellow Bermuda prr and L 
365. Inheritance: In A. cepa Fj crosses 
resistance is incompletely dominant. 
Number of genes undetermined. 

Purple blotch, Alternaria porri, P. 
S. R.: Varieties, such as Yellow Globe 
Danvers and Red Creole, that have a 
covering of wax or “bloom” on the 
foliage are more resistant than vari- 
eties with a somewhat glossy foliage 
such as Sweet Spanish. Inheritance: 
Resistance is correlated with waxy 
(nonglossy) foliage. Waxy type of foli- 
age dominant; probably monogenfe. 

Smudge, Colletotrichum circinans. P. S, 
R.: Red, yellow, and brown varieties, 
such as Southport Red Globe, Yellow 
Globe Danvers, and Australian Brown, 
are highly resistant. Inheritance: Re- 
sistance Is perfectly correlated with dry 
scale color. Three pairs of genes are 
involved in the development of red, 
yellow, and white bulb color: C-c, a 
basic color factor, the dominant C gene 
being necessary for the development of 
any pigment, consequently all cc plants 
produce white bulbs; R-r, in the pres- 
ence of C, the dominant R gene is re- 
sponsible for the production of red 
pigment — its allele r is responsible for 
yellow; I-i, an inhibiting factor I is 
partially dominant over i — all II 
plants produce white bulbs. 

Smut, Urocystis cepulae. P. S. R.: AU 
Hum fistulosum. Inheritance: In species 
crosses between A. fistulosum and A. 
cepa the Fi is intermediate in resistance 
but sterile. A fertile amphidiploid, 
Beltsville Bunching, has considerable 
resistance. 

Yellow dwarf, Af armor cepae. P. S. 
R.: Allium fistulosum. Nebuka and 
Beltsville Bunching are immune from 
all strains of yellow dwarf tested. Lines 
immune from the common strains of 
yellow dwarf are Burrell’s Sweet Span- 
ish, Colorado No. 6, Utah Sweet Span- 
ish, White Sweet Spanish, Yellow 
Sweet Spanish, Crystal Grano, Early 
Grano, Early Yellow Babosa, White 
Babosa, Crystal Wax, Lord Howe 
Island, San Joaquin, and Yellow Ber- 
muda. Inheritance: Undetermined, 



212 


YEARBOOK OF AGRJCULTURi 1B53 

Pea 


Powdery mildew, Erysiphe polygoni, 
O. S. R.: Stratagem variety. P. S. R.; 
Stratagem variety. Inheritance: Re- 
sistance is monogenic recessive. 

Septoria blotch, Septoria pisi. O. S. 
R.: One strain of Perfection and an 
introduction from Puerto Rico. P. S. 
R.: Perfection, if strain is still available. 
Inht^ritance; Not known. 

AVilt, Fusarium oxysporum f. pisi race 
I. O. S. R.: Commercial varieties 
Alcross, Wisconsin Early Sweet, Wis- 
consin Perfection, and many others. 
P. S. R.; Wilt-resistant Alaska, wilt- 
resistant Early Perfection, wilt-resist- 
ant Perfection, and many others. 
Inheritance: Monogenic dominant. 

Near wilt, Fusarium oxysporum f. 
pisi race 2, O. S. R.: Progeny in E. J. 
Delwiche nursery. P. S. R.: Dclwiche 
Commando variety. Inheritance: 
Monogenic dominant. 

Yellow bean mosaic, bean virus 2. 

O. S. R.: Wisconsin Perfection. P. S. 
R.; Wisconsin Perfection, Inheritance: 
Unknown. 

Peanut 

Leaf spot, Cercospora atachidicola and 
C. personata. O. S, R.: No significant 
resistance in any cultivated variety 
examined. Arachis marginata and cer- 
tain other wild species af)pear to be 
immune, but crosses with varieties of 
A. kypogaea have not been obtained. 

P. S. R.: None. Inheritance: Not 
known. 

r^ppERS (Capsicum annuum) 

Bacterial spot, Xanthomonas vesica- 
ioria. O. S. R.: Commercial varieties 
W'altham Beauty, Oshkosh, Sunny- 
brook, Squash, Harris Early Giant 
(some strains only), Wonder (some 
strains only), Harris Earliest (some 
strains only), Cayenne (selections), 
Santaka (selections). P. S. R.: Those 
listed under O. S. R. Inheritance: 
Both monofactorial dominant resist- 
ance gene and multiple factors arc 
suggested. There arc no clear-cut data. 


Bacterial wilt, Psewlmanas ioAm* 
acearum. O. S. R.: Ornamental variety, 
grown locally on island of Oaho, 
Hawaii, is highly resistant but not 
immune. P. S. R.: Ornamental variety 
only. Inheritance: Undetermined, 
Root rot, Phytophthora capsid. O. S. 
R.: Oakview Wonder is reported 
resistant under field conditions. P. S. 
R.: Oakview Wonder. Inheritance: 
Unknown. 

Southern blight, Sclerotium rolfsii. 

O. S. R.: Capsicum Jrutescens var. 
Tabasco is highly resistant; C. annuum 
var. Santaka is moderately resistant. 

P. S. R.: Varieties Tabasco and San- 
taka. Resistance is being incorporated 
into commercial varieties, primarily 
Bell and Pimiento typc.s. Inheritance: 
Undetermined. 

Tobacco etch virus, Marmor erod- 
ens. O. S. R. : Selections from Elephant 
Trunk x World Beater; Red Cherry, 
tolerant; P. E. I. 159241 highly re- 
sistant, possibly immune. P. S. R.: 
Varieties listed above, and resistance 
is being incorporated into commercial 
varieties, mainly Bell and Pimiento 
types. Inheritance: Undetermined. 

Wilt, Fusarium annuum. O. S. R.: A 
local variety in New Mexico and Mex- 
ican and Peruvian varieties. P. S. R.: 
Chili No. 9 and College No. 6 were 
developed by the New Mexico Agri- 
cultural Experiment Station, and the 
varieties Cristal, Nora dc Murcia, and 
Cacho dc Cabra were developed in 
P^ru. Inheritance: Undetermined. 

Root knot, Meloidogyne spp. O. S. 
R.; Varieties Anaheim Chili, Italian 
Pickling, Santaka, Cayenne (selec- 
tions). P. S. R.: Varieties under O. S. 
R. Inheritance: Unknown. 

Hawaiian pepper virus (identity 
unknown; distinct from tobacco mo- 
saic). O. S. R.: Hawaiian variety 
Waialau highly resistant; Red Chili, 
Small Chili, and Tabasco arc tolerant, 
P. S. R.: Varieties listed under O. S. 
R. Inheritance: Unknown; probably 
mulitplc factor. 

Puerto Rico mosaic virus (identity 
uncertain; po.ssibly related to potato 
mild mosaic, distinct from tobacco 



213 


$OME souices Of iiSI 

mosaic). O. S. R.: Native pepper of 
Puerto Rico and Cuaresmeno variety 
from Mexico, P. S. R.: California 
Wonder types. Inheritance; Resistance 
is monogenic. 

Tobacco mosaic, Marmor tahacu O. 
S. R.: Capsicum annuum and varieties 
World Beater No. 13, Elephant Trunk, 
Hungarian Paprika, Fresno Chili, Cap* 
sicum frutesems, and Tabasco. P. S. R.: 
Commercial types of most varieties 
are being developed. Inheritance: 
Single dominant gene for necrotic 
flecking and leaf abscission. 

Potatoes 

Blackleg, Erwinia phytophthora. O. 
S. R.: Weak resistance in £urop>e ex- 
ists in varieties Flava, Prisca, Robusta, 
and Starkeragis, P. S. R.: Original 
source and related seedlings. Inherit- 
ance; Unknown. 

Brown rot or southern bacterial 
WILT, Pseudomonas solanacearum. O, S. 

R. : Weak resistance exists in Katahdin 
and Sebago, P. S. R.: Original sources. 
Inheritance: Unknown, 

Ring rot, Corynebacterium sepedoni* 
cum. O. S. R.; President, Friso, Teton, 
and United States Department of Ag- 
riculture seedlings 46952 and 055. 
P. S. R.: President, Furore, Teton, 
Saranac, Seedling 46952, and a num- 
ber of seedling varieties related to 
these. Inheritance: Unknown. Five 
resistant varieties selfed; 55.4 to 85.0 
percent of seedlings resistant. 

Common scab, Streptomyces scabies. 
O. S. R.r European varieties, Jubel, 
Arnica, Hindenburg, Rheingold, Ac- 
kersegen, and Ostragis. P. S. R.: Pre- 
ceding varieties and American varie- 
ties, Ontario, Menominee, Seneca, 
Cayuga, Yampa, Cherokee, and re- 
lated seedling varieties; also wild 
species Solanum commersoniij S. chacoense, 

S. caldasii var. glabrescens and S.jamesii, 
Inheritance: Tetrasomic. There is one 
gene difference in some crosses; appar- 
ently more than one in others. Degree 
of resistance depends on dosage of 
resistance genes. 

Late blight, Phytophthora injestans. 


TAHCE IN CROP PLANTS 

O. S. R. : Solanum demissum . immune 
from all known races. S. andreanum^ S. 
ajuscomscy S. henryi, S. antipovichi, S. 
milaniy S, polyadenium, S. vallis-mexici^ S. 
verrucosum probably immune. W varie- 
ties from Germany, probably related 
to S. demissum^ are resistant to field 
pees. P. S. R.; Original sources. There 
is immunity from certain races in 
Essex, Ashworth, Placid, and seedling 
varieties related to Solanum demissum 
and Kennebec, Cherokee, and other 
named and numbered varieties de- 
scended from German W varieties. 
Inheritance: Tetrasomic polygenic, 
immunity dominant. Three, four, or 
more genes combined to produce im- 
munity to all known races. Number of 
genes is determined by the reaction of 
different seedlings to different physi- 
ologic races of the organism; mode of 
inheritance of individual genes deter- 
mined by ratios found in selfed lines. 

Verticillium wilt, Verticillium albo* 
atrum. O. S. R.: Menominee, Sequoia, 
Saranac, United States Department 
of Agriculture seedling varieties 41956, 
792-88, X 528-170, B986-8, B595-76, 
and European varieties Libertas, Vo- 
ran, Furore, Aquila, Friso, Iduna, 
and Populair. P. S. R.: Original 
sources and related seedling varieties. 
Inheritance: Unknown. 

Wart, Synchytrium endobioticum. 
O. S. R.: Snowdrop, Great Scott, 
Jubel, Hindenburg, and other Euro- 
pean varieties, and the American varie- 
ties Green Mountain, Irish Cobbler, 
Triumph, Spaulding Rose, and Bur- 
bank. P. S. R.: In at least 80 Euro- 
pean varieties and in Green Mountain, 
Irish Cobbler, Triumph, Spaulding 
Rose, Burbank, Katahdin, Sequoia, 
Pawnee, Ontario, Kennebec, Calrose, 
Mohawk, Chisago, Mesaba, and a 
number of seedling varieties. Inherit- 
ance; Tetrasomic. One gene difference 
in some crosses and more than one in 
others. Dominant gene X giving 
immunity even in simplex. Genes Y 
and Z complementary conditioning 
immunity when both present even in 
simplex. 

Aphid injury. O. S. R.: Segregates 



YEARBOOK OF AGRICULTURE 1953 


214 

of a cross between Houma and 
United States Department of Agri- 
culture seedling variety 96-56. P. S. R: 
Same as original. Inheritance: Un- 
known. 

Hopperburn. O. S. R.: Solanum poly- 
adenium^ S, macolae, S, commersoniiy S, 
chacoense^ S, caldasii^ Rural, Sequoia, 
Hindenburg, Jubel, Katahdin, and 
Sebago. P. S. R.: Original species 
and varieties and seedling selections 
related to the original varieties. In- 
heritance: Unknown. 

Leaf roll. O. S. R.: Degree of re- 
sistance exists in Houma, Katahdin, 
Triumf, Jubel, Flava, Imperia, Kep- 
plcstone Kidney, and Aquila. No 
varieties are immune. Immunity prob- 
ably in Solanum chacoense and S. andi- 
genum; tolerance in S. polyadenium. 
P. S. R.: Preceding varieties and 
species and in the United States 
Department of Agriculture seedlings 
X 927-3, X 1276-185, B 24-58, 
B 579~3, and other related seedlings. 
Some are more resistant than original 
varieties. Inheritance: Tetrasomic. 
Probably several genes. Katahdin and 
Houma probably simplex for one 
gene; X 1276-185 probably duplex. 

Net necrosis, current-season infec- 
tion by leaf roll virus. O. S. R.: Im- 
munity in Katahdin, Chippewa, and 
several other varieties. P. S. R.: 
Preceding varieties and related seed- 
ling varieti^^s. Inheritance: Unknown. 

Virus A (A+X=mild mosaic). 
O. S. R.: United States Department 
of Agriculture .seedling 24642, Irish 
Cobbler, and Spaulding Rose. P. S. R.: 
Seedling 24642, Irish Cobbler, Kaiah- 
din, Chippewa, Sebago, Kennebec, 
Earlaine, and many related seedling 
varieties. Irish Cobbler and Earlaine 
show hypersensitivity. Inheritance: 
Tetrasomic. Single dominant gene for 
field immunity. Katahdin probably 
duplex. 

Virus X (latent mosaic). O. S. R.: 
United States Department of Agricul- 
ture seedling 41956. P. S. R.: Seedling 
41956 and seedling varieties related to 
seedling 41956 and Solanum acaule,: In- 
heritance: Tetrasomic. Two comple- 


mentary genes, immunity dominant. 
Seedling 41956 heterozygous; some 
plants of S, acaule probably homozy- 
gous; others heterozygous. 

Virus Y, vein banding or vein clear- 
ing, (X+Y=rugosemosaic).O.S. R.: 
Hypersensitivity in Solanum simplici- 
folium^ S. salamanii, S, demisstim^ S. 
rybiniiy and accession Nos. 25941 and 
25942. Immunity in S. chacoense^ S, 
cordobense, S, garciae, S. macolae, S, 
ajuscoense^ S. antipovichi^ S. polyadenium^ 
and probably 5 . commersonii and S, 
chaucha. Field immunity in accession 
Nos. 25830 and 25832. Weak resistance 
in Katahdin and Chippewa. P. S. R.: 
In original sources and in seedling 
varieties related to them. Inheritance: 
Tetrasomic. Hypersensitive reaction 
depends on one or more recessive 
genes. Tolerance to virus is dominant. 

Yeli.ow dwarf. O. S. R.: Sebago, 
Russet Burbank, and Irish Cobbler are 
resistant but not immune. P. S. R.: 
Sebago, Russet Burbank, Irish 
Cobbler. Inheritance: Unknown. 

Spinach 

Blue mold, Peronospora effusa. O. S. 
R.: United States Department of 
Agriculture collection, P. E. I. 140467, 
made in Iran in 1940. P. S. R.: Com- 
mercial types are now being developed. 
Inheritance: Resistance is a monogenic 
dominant. 

Fusarium wilt, Fusarium oxysporum 
spinaciae. O. S. R.: Commercial Vir- 
ginia Savoy. P. S. R.: Resistant selec- 
tion of Virginia Savoy was developed 
by the Virginia Truck Experiment 
Station. Inheritance: Not determined. 

Blight, cucumber virus i. O. S. R.: 
Wilt plant oiSpinacia oleracea, collected 
near Liaoyang in North Manchuria in 
1918. P. S. R.: Commercial varieties 
Virginia Savoy and Old Dominion, 
Inheritance: Resistance is monogenic 
dominant. 

Sweetpotato 

Stem rot (wilt), Fusarium hyper oxy- 
sporum and F, oxysporum f. batatas. 
O. S. R.: Selections from open- 



SOME SOURCES OF RESISTANCE IN CROP PLANTS 


pollinated seedlings of Cuban variety 
Americano; P. E. I. 153655, intro- 
duced from Tinian Island in 1946; 
Triumph, a white-flesh American va- 
riety; and Japanese white-flesh varie- 
ties Norin No. 2, Norin No. 3, Taihaku 
Saitama No. i. P. S. R.: Goldrushand 
numerous seedling selections now 
being developed. Inheritance: Mul- 
tiple factor. 

Tomato 

Bacterial canker, Corynehacterium 
michiganense. O. S. R.: Some collec- 
tions of Lycopersicon pimpinellijolium, 
P S. R.: L. pimpinellijolhim. No com- 
mercial varieties. Inlieritance; Un- 
known. 

Bacterial wilt, Pseudomonas solana^ 
cearum, P. S. R.: Puerto Rico pear and 
some L. pimpinellifolium. P. S. R.: Pre- 
ceding varieties and commercial types 
now being developed. Inheritance: 
Unknown. 

Collar rot, Alternaria solani, O. S. 
R.: Devon Surprize and some other 
European forcing vaiieties. P. S. R.: 
Southland, Urbana, and other com- 
mercial types under development. 
Inheritance: Monogenic; fluctuating 
dominance. 

Early DLiarr, Alternaria solani. O, S. 
R.: Devon Surprize and some oilicr 
European forcing varieties. P. S. R.: 
Preceding varieties and commercial 
types now Ix-ing develc)])ed. Inherit- 
ance: Probalfly two factors or more. 

Fusarium wilt, Fusarium bulbigermrn 
var. lycopersici. O. S. R.: P. E. I. 79532 
from Peru and other collections of 
L. pimpinellifolium. P. S. R.: Pan 
America, Sunray, Southland, Fortune, 
Jefferson, Golden Sphere, and Mana- 
hill. Inheritance: Monogenic; near- 
immunity partially dominant. 

Gray leaf spot, Stemphylium solani. 
O. S. R.; L. pimpinellifolium. P. S. R.: 
L. pimpinellifoltum and commercial 
types now being developed. Inherit- 
ance: Monogenic; resistance domi- 
nant, linked with wilt immunity. 

Late blight, Phytophthora injestans. 
O. S. R.: Low-level resistance in 
several wilt types of L. esculentum, i. e., 


215 

P. E. I. 134208 from India. P. S. R.: 
Low-level resistance in Garden State 
and Southland. Inheritance:, Two 
small-fruited tomato types used as 
ornamentals and designated as Pi and 
Pa were observed to have a high degree 
of resistance in southern Florida. In- 
heritance segregation ratios in crosses 
made with cultivated varieties indi- 
cated that resistance was due to one 
main factor and one or more modify- 
ing factors. Fa stocks uniformly resist- 
ant at Homestead, E'la., w^ere not 
resistant in the high valleys of North 
Carolina or at Huttonsville, W. Va. 
The reason for the difference has not 
been determined. 

Leaf mold, Cladosporium fulvum. 

O. S. R.; L. pimpineilijolium and L. 
hirsutum. P. S. R.: Vetoinold, Im- 
proved Bay State, Quebec 5, and 
Globelle. Inheritance; Polygenic dom- 
inance incomplete. Multiple strains 
of causative organism. 

Septoria leaf spot, Septoria lycoper^> 
ski. O. S. R.: L. hirsutum^ P. E. I. 
127827 from Peru, and T6~02-M6. 

P. S. R.; Preceding varieties and com- 
mercial types now be;ing developed. 
Inheritance: Resistance partially dom- 
inant. 

Vertictlliitm wilt, Verticillium albo~ 
atrum. O. S. R.: Peru wilt, L. pimpinel^ 
lifoliurn, and Utah accession No. 665. 
P. S. R.; Riverside, Essar, Simi, VR 4, 
and VR 1 1. Inheritance: Resistance is 
monogenic dominant. 

Curly top, Ruga verrucosans. O. S, 
R.: L. per uvianum var. denlalum; P. E. I. 
128660, collected in Tacna, Peru, 
1938; L. chilense: L. pissisi; P. E. I. 
127829, collected between San Juan 
and Magdalena, Peru, 1938; Z.. gland- 
ulosiwi; P. E. 1 . 126440, collected be- 
tween Vangos and Canta, Peru, 1938* 
and Red Peach. P. S. R.; Original 
introductions and commercial types 
now being developed. Inheritance: 
Not detennined. 

Mosaic, M armor tabaci. O. S. R.; 
Zi. hirsutum. P. S. R.: L. hirsutum; no 
commercial varieties. Inheritance: 
Unknown. 

Root knot, Meloidogyne incognita. 



YEARBOOK OF AGRICULTURE 1953 


216 


O. S. R.: L, peruvianum, P. S. R.; 
vianum and commercial types being de- 
veloped. Inheritance: Resistance par- 
tially dominant, apparently due to one 
or two major dominant genes; modi- 
fiers or additive action genes possible. 

Spotted wilt, 3 viruses. O. S. R.: 
L. pimpinellijolium^ L, peruvianum^ and 
California BC 10. P. S. R.: Pearl 
Harbor, Manzana, German Sugar, 
Oahu, Lanai, Hawaii, Maui, Molokai, 
Kaudi, and Nuhau. The last 7 of these 
varieties probably inherited their re- 
sistance from German Sugar and L. 
peruvianum. Inheritance: Resistance 
from Pearl Harbor to Hawaii strain of 
spotted wilt is a monogenic dominant. 

Watermelon 

Anthracnose, CoUetotrichum lagena- 
rium. O. S. R.: Native African melons. 

P. S. R.: Congo. Inheritance: Mono- 
genic, partially dominant. 

Downy mildew, Pseudoperonospora 
cubensis, O. S. R.: Santo Domingo 
melons. P. S. R, : None. Inheritance: 
Unknown. 

Wilt, Fusarium niveum. O. S. R.: 
Probably citron P. S. R.: Hawkes- 
buiy, Leesburg, Blacklee, Klondike 
R7, etc. Inheritance: Unknown; prob- 
ably polygenic. 

Frederick J. Stevenson has been 
employed smee ig^o as a geneticist in 
charge 0/ the national potato-breeding pro^ 
gram of the Department of Agriculture, 
From igig to 1925^ at the State College of 
WashingtoHy he worked with breeding for 
resistance to bunt in wheat and smut in oats. 
From ig2^ to igjOy at the University of 
Minnesota, he cooperated with others in 
breeding for resistance to rusts in wheat and 
oats and Helminthosporium in barley, 

Henry A. Jones has been with the 
Department of Agriculture since iggS, 
Previously he was head of the division of 
truck crops at the University of California, 
His chief investigations have had to do with 
the development of hybrid onions and disease 
resistance in onions. He is a graduate of the 
University of Nebraska and the University 
of Chicago, and in ig^2 received an honors 


ary degree of doctor of science from the 
University of Nebraska, 

Many persons^ who are among the leaders in the 
efforts to breed disease-resistant plants, gave them 
the information regarding the crops with which 
they work. They are: 

Alfalfa, O, S, Aafnodt; Apples, J, R, Shay; 
Apricots, C. O. Hesse; Asparagus, G. C, Hanna; 
Barley, G. A, Wiebe; Beans and Lima Beans, 

R. D. Thomas, R. E. Wester, and W. J, Zau- 
meyer; Celery, H. M, Afunger; Clovers, E. A. 
Hollowell; Corn, M. T, Jenkins; Cotton, G. J. 
Harrison; Cowpeas, Helen Sherwin; Cranberry, 
A. C. Goheen; Crucifers , J. C. Walker; Cucum- 
ber, S. P. Doolittle; Flax, fiber, D. W. Fishier; 
Flax, grain, H. H. Flor; Flowers, snapdragons, 

S. L, Emsweller; Forestry, Lee M. Hut^ins; 
Grapes, H. C. Barrett, N. H. Loomis, H. P. 
Olmo, and C. F. Williams; Grasses, J. R. Harlan, 
M. A. Hein, W. R. Kneebone, and K. W. Kreit- 
low; Hops, K. R. Keller; Lettuce, R. C. Thomp- 
son and T. W. Whitaker; Muskmelons, H. M. 
Munger and T. W. Whitaker; Nut crops, J. W. 
McKay; Oats, H. C. Murphy; Onions, H. A. 
Jones; Pea, J. C. Walker and W. J. Z<^umeyer; 
Peach, J. H. Weinberger; Peanut, B. B. Higgins; 
Pear, J. R. Kienholz; Peppers, P. G. Smith; 
Peppermint, M.J. Murray; Potatoes, F. J. Stev- 
enson; Rice, J. W. Jones; Safflower, C. E. Claas- 
sen; Sorgo, 0, H. Coleman; Small fruits, G. M. 
Darrow and D. H, Scott; Southern legumes, 
P. R. Henson; Soybeans, M. G. Weiss; Spear- 
mint, M. J. Murray; Spinach, G. S. Pound; Stone 
fruits, L. C. Cochran; Sugar beets, Eubanks Cars- 
ner, F. V. Owen, J. S. McFarlane, and Dewey 
Stewart; Sugarcane, E. V. Abbott; Sweetpotatoes, 
C. E. Steinbauer; Tobacco, E. E. Clayton; To- 
mato, C. F. Andrus, O. S. Cannon, W. A. Frazier. 
J. W, Lesley, and J. T. Middleton; Wheat, 
E. R, Ausemus, B. 8. Bcyles, and E, C. Stak- 
man; and Watermelon, C. r. Andrus, 



1 

Rhizoctoaia leaf spot on cotton. 




The Many 
Ailments 
of Clover 

Earle W, Hanson, Kermit W. Kreitlow 

There are some 250 described species 
of true clovers { Trifolium) but only four 
species — red, alsike, white (including 
Ladino), and the crimson — are widely 
grown and of great importance. 

The sweetclovers {Melilotus) arc not 
tiue clovers. Twenty- two species of 
swcctclover are recognized. Three 
species — ^white, yellov , and sour clo- 
ver — are of importance in agriculture. 

All clovers arc subject to injury from 
diseases. All parts of the plant are 
attacked and sometimes destroyed — 


the roots, crowns, stems, leaves, and 
inflorescences. Fungi, bacteria, and the 
viruses all can damage the clovers. 

Some of the pathogens infect only 
specific organs of the plant, such as the 
leaves or roots. Others attack several 
or all parts of a plant. The pathogens 
differ also in parasitism. Some infect 
only certain .species of clover. Others 
have a broad range of hosts and can 
attack nearly all clovers and many 
other hosts as well. 

An important problem in produc- 
ing clovers, the establishment and 
maintenance of stand, involves several 
factors. One is the root and crown 
disease complex, w^hich includes the 
seedling blights, root rots, and crown 
rots. 

Those diseases probably are the most 
important of all clover diseases. They 
occur wherever clovers are grown. 
They are caused by a complex of 
soil inhabiting fungi. The fungi may 

217 



2i8 


YEARBOOK OF AGRICULTURE 1953 


be widely distributed or occur only lo- 
cally. Some are virulent pathogens that 
can attack vigorous plants. Others are 
weak pathogens that cause damage 
only after the plants have l>een weak- 
ened by w inter injury, nematodes, in- 
sects, drought, unfavorable soil con- 
ditions, or improper management. 
Some are primarily seedling pathogens. 
Others attack clover plants of any age. 
Some are primarily root pathogens. 
Others arc primarily crown pathogens. 
Several organisms may attack a plant 
simultaneously, or one may follow 
another in sequence. Thus the difficult 
problem of root and crown disease is 
one that must receive greater atten- 
tion if productive stands are to be 
maintained. 

Crow'n wart of clover, caused by 
Vrophlyctis tri/olii, occurs in central 
Europe on red clover (Irijolium pra- 
tense) ^ white clover ( 1 \ repens), and 
some others. In the United States the 
disease is of minor importance. It 
occurs mainly in the South Central 
States and on excessively wet soils. It 
is similar to the more important crown 
wart of alfalfa. Its characteristic symp- 
tom is the formation of irregularly 
shaped galls around the crown of the 
plant, at and just below the soil level. 
The galls first loeconie noticcal.de in 
late spring and increase in size as 
summer advances. Infected plants wilt 
in hot weather. Leaves of wlutc clover 
arc sometimes distorted. 

Sclerotinia crown and stem rot is 
caused by Sclerotinia trijoliorum, and is 
widely distributed, especially in the 
regions of mild winters or heavy snow 
cover. It has Jong been recognized as 
one of the most destructive diseases 
of clover in northern Europe. It occurs 
also in the Soviet Union and Canada. 
In the United States it is of consider- 
able economic importance in the 
southern and central clover belts and 
causes extensive damage in the Pacific 
Northwest and in the Northeast. 
Rarely does it occur in the north central 
part of the northern clover belt. The 
disease spreads and develops most 


rapidly during cool — 55® to 65® F. — 
wet w'cather, but the fungus that causes 
it can grow and infect plants at 
temperatures ranging from below 
freezing to 75®. 

Sclerotinia trijoliorum has a broad 
range of hosts, which include all 
important true clovers and the sweet- 
clovers, alfalfa {Medicago sativa), black 
medic (A/, lupulina), birdsfoot trefoil 
(Lotus corniculalus) , sainfoin (Onobrychis 
viciaejolia) , and many other legumes 
and nonlegumes, including numerous 
weeds. Red clover, crimson clover 
(Trifolium incarnatum) , and alsike clover 
(T. hybridurn) are all very susceptible. 
White clover is generally considered 
to be less susceptible but not immune. 
Some other species of Sclerotinia may 
also occasionally infect clovers. 

The disease is commonly referred 
to as a crown and stem rot, but it can 
atUck all parts of the plant. Symptoms 
first appear in the fall as small, browm 
spots on the leaves and petioles. The 
heavily infected leaves turn grayish 
brow^n, wither, and become overrun 
with white mycelium, which spreads 
to the crowns and roots. By late 
winter or early spring the crowms and 
basal parts of the young stems show 
a brow n, soft rot, which extends down- 
ward into the roots. Con.sequently 
part or all of the new growth of the 
infected plants wilts and dies. Stolons 
of Ladino clover may become .soft and 
flaccid over their entire length or only 
small areas may be affected. 

As the .Sterns and petioles arc killed, 
a mass of w hite mycelium grows over 
them. Some of the mci.sses of mycelium 
then change into .small, hard, black, 
cartilaginous liodies — the seJerotia. 
They are attached to the surface of 
(or iml^cdded in) the dead stems, 
crowns, and roots or in the soil near 
the roots. Some are as small as a 
clover seed. Some arc larger than a 
pea seed. 

When the affected plant parts decay, 
the sclcrotia remain in the soil as a 
future source of infection. Sclcrotia 
are the chief means by which the 
fungus survives from year to year. 



THE MANY AILMENTS OF CLOVEI 


They can remain viable in soil for 
several years. In the fall, if conditions 
are right, the sclerotia germinate and 
produce one or more small, disk- 
shaped, pinkish- buff, mushroomlike 
fruiting bodies called apothecia, which 
arc borne individually on slender 
stalks. The apothecia are one-sixteenth 
to one-fourth of an inch in diameter. 
They produce millions of spores, which 
spread to the leaves and petioles of 
nearby plants, causing infection and 
repeating the cycle. 

The greatest reduction in stands 
occurs in late winter and early spring. 
The damage therefore is sometimes 
confused with winter killing. The dis- 
ease usually occurs in patches through- 
out a field, but when conditions are 
very favorable the patches may become 
so numerous as to merge and cause 
extensive damage to a stand. In the 
South a few days of warm weather 
sometimes checks the disease, and the 
plants recover. 

Control is difficult. Clean cultivation, 
deep plowing to bury the sclerotia be- 
yond their capacity to send up apothe- 
cia, and long rotations are helpful. 
Care should be taken not to distribute 
the sclerotia with clover seed. Grazing 
or clipping in late fall sometimes re- 
moves infected leaves and reduces the 
amount of foliage that may become 
infected and mat down on the crowns 
during the winter. Adapted varieties 
arc more resistant than non adapted 
strains. The most promising method of 
control appears to be the breeding of 
resistant strains. 

Common root rot is a group of root 
diseases caused by species of Fusarium 
and several other soil fungi that pro- 
duce similar symptoms and frequently 
attack plants simultaneously. The rela- 
tive prevalence and importance of the 
fungi vary with the locality, kind of 
clover, age of plant, season of the year, 
soil type, and management practices. 
Mostly they arc weak pathogens and 
cause damage after the plants have 
been weakened or injuicd. Most of 
them are widely distributed and cause 
damage wherever clovers arc grown. 


219 

Scientists have learned a lot about 
the problem, but relatively little is 
understood concerning many of its 
phases. The field symptoms of common 
root rot of red clover are well known, 
for example, but attempts to repro- 
duce them under controlled conditions 
often are unsuccessful. Research men 
have demonstrated that some isolates 
of the fungi can attack clover seedlings. 

Species of Fusarium have been most 
frequently reported as causing root rot. 

Symptoms of the disease are a local- 
ized or general rotting of any part of 
tlic root system. Taproot, secondary 
roots, and even the crown may be 
attacked. The color of the diseased 
areas ranges from light brown to black. 
The rotting may be limited to the cor- 
tical areas around the exterior of the 
root, the vascular core may be dis- 
colored, or the entire root may be 
affected. Secondary roots are con- 
stantly pruned away by the rots and 
new secondary roots are formed to 
compensate, but the replacement proc- 
ess is usually the slower, so that by 
the end of the second year most plants 
have left only a few short secondary 
roots. The lower part of the taproot 
often is destroyed completely. Such 
destniction causes wilting and a grad- 
ual dying of the plant. 

Common root rot kills plants in all 
stages of development. Effects on stand 
arc most conspicuous during the second 
year, but losses up to 45 percent during 
the first year are not uncommon. Stand 
losses occasionally occur in the spring 
when the plants are weak because of 
low food reserves or winter injury. 
Diseased stands frequently produce a 
fair first crop of hay but fail to recover 
and to produce a second crop. Most 
clovers and sweetclovert; are susceptible 
to root rot. 

Besides Fusarium^ Rhizoctonia^ Phoma^ 
and other organisms may be associated 
vrith root rots. Plenodomus meliloti and 
Cyliudrocarpon ehrenhergi are of primary 
importance following the winter dor- 
mancy period on sweetclover in Al- 
berta, Canada. 

Control is difficult, but any practice 



220 


YEARBOOK OP AGRICULTURE 1953 


that improves the general vigor of the 
plant is helpful. Proper liming, ferti- 
lization, and crop rotation arc impor- 
tant. Only adapted varieties should be 
grown. No varieties available in 1953 
had high resistance wheh conditions 
favor the disease. Plant breeders have 
under way a project to develop resist- 
ant strains of red clover. 

Phytophthora root rot, caused by 
Phytophtnora cactorum^ is a widespread 
disease of sweetclover in North Amer- 
ica, notably Ohio, Indiana, Illinois, 
and Missouri. It occurs in All3erta and 
Ontario. The fungus attacks individ- 
ual plants or small groups of plants in 
fields or along roadsides. It is most 
abundant in low, wet parts of fields, 
where in seasons of heavy spring rain- 
fall and cool temperatures it may kill 
most of the plants. 

Its presence is first noted in the 
spring, when infected plants wilt, die, 
or are generally unthrifty. When their 
roots arc examined, the upper portions 
usually are found to be rotted. The 
decay generally is limited to the upper 
3 or 4 inches but may extend as much 
as 8 inches below the crowm. The 
decayed places usually are soft and 
watery. The color changes but little at 
first. Later they may become dis- 
colored and shrunken. 

Crop rotation and the use of well- 
drained fields are helpful control meas- 
ures. It should also be possible to 
develop resistant varieties l>ecausc 
resistant plants are known to exist. 

Seedling blights, caused by Pythiuniy 
Rhizoctomoy and other fungi, are the 
most destructive seedling diseases. 
They occur wherever clovers arc 
grown. Sometimes they seriously re- 
duce the stands. Three types of injury 
occur. Preemergence killing starts 
shortly after the seed is sown and de- 
velops rapidly, so that the seedlings are 
destroyed before they emerge from 
the soil. In postemergence damping- 
off, infection commonly occurs before 
emergence, although the rate of/Jisease 
development is slower and the seed- 
lings emerge only to be killed soon 
thereafter. Root and hypocotyl rotting 


causes varying degrees of stunting, in 
which plants survive the early seed- 
ling stages, after which some recover 
and some die. 

Seedling blights are caused by a 
complex of fungi, including several 
species of Pythiuniy Rhizoctonicy FusO’- 
riuniy Gliocladiuniy Phomay and others. 
One of the most virulent is Pythium 
debaryanum. It would seem that seed 
treatment might help control this 
group of diseases, but the results of 
field tests have given limited encour- 
agement for this method of control. 

The stem diseases attack the support- 
ing and conducting systems of the 
plant. Often they cause serious losses. 
Usual symptoms are stem discolora- 
tion, withering and dying of attached 
leaves and petioles, and general wilt- 
ing and stunting of the plant. Fre- 
quently stems break off or crack open 
at the site of infection. Several of the 
major diseases of clover are included 
in this group. 

Northern anthracnosc, caused by 
Kabatiella caulivoray is a major disease 
of red clover in the cooler areas of 
North America, Europe, and Asia. It 
develops l:>cst at 68® to 77® F. and is 
checked by continuous hot dry weather. 
In the United States it is important 
only in the northern clover regions but 
there it frequently causes damage — 
occasionally exceeding 50 percent of 
the crop in some fields. Losses as high 
as 50 to 60 percent have been reported 
in Germany. Complete crop failures 
have been observed in the Nether- 
lands. Seed production and hay yield 
and quality are greatly reduced in 
badly infected fields. 

The disease is serious only on red 
clover. It may occur on alsike, white, 
crimson, and Persian (Tri/olium wm- 
pinatum) clovers and possibly others. 
It has never been found on alfalfa, but 
has been reported on black medic and 
on sainfoin in the Netherlands. The 
species of fungus inciting the disease 
consists of a large number of physio- 
logic races, which differ in their capac- 
ity to infect different species of clover 



THE MANY AILMENTS OF CLOVER 


221 


and different strains of a particular 
species. No red clover strain yet de- 
veloped is immune to the disease, but 
wide differences in resistance exist 
among European and American 
strains. Varieties developed in the 
southern part of the United States are 
more susceptible than those developed 
in the northern clover areas. 

Symptoms are confined mostly to the 
petioles and stems. Infection also 
occurs on the petiolules — small stalks 
connecting the leaflets to the petiole — 
and occasionally on the leaflets them- 
selves. The first symptoms noticed in 
the field are usually dark-brown or 
black spots on the petioles. The spots 
soon cut off translocation to the parts 
above them — the upper part of the 
petiole and the leaf — causing them to 
wilt, turn grayish brown, and die. The 
petiole bends downward at the site 
of the lesion to form the familiar 
“shepherd’s crook.” Stem lesions are 
most characteristic. They develop first 
as small, dark spots, which soon 
lengthen to form lesions with dark 
margins and light-colored centers. As 
the stem grows, a crack often appears 
in the center of the lesion. Stems 
finally may be girdled and killed. 
Plants in a badly infected field look as 
if they were scorched with fire, because 
of the abundance of blackened and 
broken stems, withered petioles, and 
brown, dead leaves. The name scorch 
has been aptly used in Britain to de- 
note the disease. 

Southern anthracnose, caused by 
ColUtotrichum irifolii, is a major disease 
of red clover in the southern clover 
belt of the United States. It has been 
recorded as far north as southern 
Canada, but is primarily a high-tem- 
perature disease that flourishes at 
about 82® F. It is of little economic 
importance in the northern clover 
areas. It is confined mostly to North 
America, although it has been re- 
ported on alfalfa in South Africa and 
in Europe. It occurs occasionally on 
crimson clover, sub clover (Trifolium 
subtmaneum), bur-clover (Meilicago his^ 
pida)^ and white swcetclover (Aff/i- 


lotus alba). It has not been observed on 
white clover. Alsike clover is prac- 
tically immune. 

Southern anthracnose has been re- 
garded as the most destructive disease 
of red clover in the Southern States. 
It reduces yields of hay and seed and 
can destroy stands of clover. A re- 
sistant variety, Kenland, is available. 
Most European and American strains 
developed in regions where the disease 
does not occur are susceptible; hence 
it is important to grow only locally 
adapted strains or strains known to be 
resistant. 

Symptoms resemble those of north- 
ern anthracnose; in fact, a positive 
identification in the field is frequently 
difficult and sometimes impossible. 
Dark tufts of setae in the older lesions 
indicate that the disease is southern 
anthracnose. But there are other dis- 
tinguishing features. Southern an- 
thracnose commonly attacks the upper 
part of the taproot; that has not been 
observed for northern anthracnose. 
Southern anthracnose usually pro- 
duces more spotting of the leaves, but 
that is not an infallible characteristic 
because of the frequent presence of 
similar leaf spots incited by other 
pathogens. Like northern anthracnose, 
it may occur on plants at any stage of 
development. It most commonly de- 
velops on the young, succulent parts 
of stems and petioles but is not limited 
to them. 

The disease occurs on the leaves as 
dark-brown spots of irregular shape, 
which vary from pin-point lesions to a 
general infection over most of the sur- 
face. Petioles are very susceptible. 
They become dark brown, and the 
attached leaflets droop. First symp- 
toms on the stems and petioles are 
small, water-soaked spots, which usu- 
ally lengthen to form long, depressed, 
dark-brown or black lesions, many of 
which develop gray or light-brown 
centers. Lesions near the base of a 
stem often cause death and browning 
of the entire stem. 

The most destructive effect of south- 
ern anthracnase is on the taproot and 



222 


YEARBOOK OF AGRICULTURE 1953 


crown. Dark lesions develop on the 
upper part of the taproot, gradually 
girdle it, and cause the plant to wilt 
and die. This crown rot is closely 
associated with taproot decay and may 
result from spread of the fungus upward 
from the roots or downward from the 
stems and petioles. Diseased crowns 
become brittle so that the stems are 
readily broken off at the soil level. 
Crown and root rot caused by southern 
anthracnose kills some plants and 
weakens others so that they cannot 
survive long drought, adverse winter 
conditions, and attacks of other dis- 
eases. 

Black stem, caused by Phoma^ Myco- 
sphaerella, and Ascochyta, is a major 
disease ot clovers. It is widely distrib- 
uted and may cause extensive damage 
during cool, wet weather in the fall, 
late winter, and spring. It causes the 
familiar stem blackening and repeated 
defoliation, which w'eakens and some- 
times destroys stands. I’He disease was 
so severe in Kentucky in 1933 on 
some of the unadapted red clovers that 
plots that had had perfect stands the 
previous December were bare by April. 

Among the fungi that catise black 
stem of clovers are Ascochyta imperfecta^ 
which occurs mostly on alfalfa but 
sometimes on clovers; Phoma trifolii, 
the organism most frequently attacking 
red clover; and Mycosphaerella lethalis^ 
the cause of black stem of sweetclovcr. 
Little is known about the host range of 
these pathogens except that each of 
them can infect alfalfa, red clover, and 
sw’cetclover and each is primarily the 
cause of the disease on its own crop. 
During midsummer and fail, another 
pathogen, Cercospora, also causes black 
stem. 

The most conspicuous symptom Is 
stem blackening, which may involve 
all or any part of the stem. Blackening 
increases when clover is not cut at the 
proper time or w'hen the crop is left for 
seed. Frequently young shoots or 
petioles are girdled and killed. This, 
as well as leaf infection, may result in 
severe defoliation. 

' On red clover the disease produces 


small, dark-brown or black spots, 
which increase slowly in size and 
eventually kill the affected parts. 
Infection occurs the first summer on 
spring-sown clover but becomes more 
destructive the following late winter 
and spring. On unadapted clovers, 
new leaves may be killed as rapidly as 
they are formed. On sweetclovcr the 
disease appears most commonly in the 
spring of the second year. The spots 
at first are dark. As they enlarge they 
change to light brown. Leaf spotting 
is increased l:>y frost injury, which 
seems to provide an avenue of entrance 
for the fungus. Heavy stands may be 
greatly injured. The disease is more 
severe on plants that have been clipped 
or grazed. 

Crop rotation and burning of dead 
leaves and sterns before new growth 
develops in the spring are helpful con- 
trol measures. Breeding for resistance 
has been started. 

Stem canker, or gooseneck, is caused 
by Ascochyta caulicola. It W’as first 
reported in Germany in r 903 as a new 
disease of sweetclovcr. It is now known 
to occur iji most areas of the world 
where sweetclovcr Is grown. It has not 
been observed on other legumes. 

It produces silvery-white cankers on 
the stems, petioles, and occasionally 
the midribs of the leaves. The cankers 
vary in size. They are stippled with 
numerous liny black dots and have 
brown margins. On the lower parts ol 
the stems tlie cankers may be so large 
and numerous as to girdle the stems. 
On the upper parts they are less abun- 
dant, smaller, and more isolated. Heav- 
ily infected stems often appear swollen, 
^arc retarded in development, and have 
fewer and smaller leaves. They also 
tend to twist and bend at the top. 

Leaf diseases usually do not kill 
plants, but they interfere with the nor- 
mal functions of the leaf. Sometimes 
they cause defoliation, which reduces 
yield, quality, and palatability of the 
forage. If the defoliation is extensive 
and continuous, the plants lose vigor, 
are less able to survive unfavorable 



THE MANY AILMENTS OF CLOVER 


conditions, and are more readily at- 
tacked by pathogens that cause root 
rots. 

Pseudopeziza leaf spot, caused by 
Pstudoptzizfl is widespread in the 

cooler, humid clover regions of the 
United States and Europe. It has been 
reported also from Canada and Russia. 
It usually is of minor importance, but 
occasionally severe local outbreaks 
cause extensive defoliation. Serious 
outbreaks have occurred in northern 
Indiana, Ohio, and the Northeastern 
States. It has been called the most seri- 
ous leaf disease of red clover in New 
York. 

It resembles the pseudopeziza leaf 
spot of alfalfa, but it does not attack 
alfalfa and the disease of alfalfa does 
not attack the clovers. It occurs on red, 
alsike, white, crimson, zigzag {Trijol- 
ium medium), and strawberry {T.Jragi^ 
ferurn) clovers and several others. A 
similar disease of sweetclovcr is caused 
by P, melUoti. 

Dark spots that may be olive to red- 
dish brown, purple, or Idack develop 
on either leaf surface. The spots are 
tiny, angular, or round and commonly 
have dendritic margins. A minute, am- 
ber, jelly like globule occurs in the cen- 
ter of the elder spots. The globules, or 
fruiting bodies, more frequently are 
found on the lower side of the leaf but 
occasionally occur on both sides. I'hey 
are most abundant in w'et weather. 
Later they dry up, shrink, become al- 
most black, and are not readily de- 
tected. Positive diagnosis of the disease 
in its early stages in the field is difficult 
because at first the minute pin-point 
lesions are not markedly different from 
those of other leaf spots. The disease is 
almost entirely limited to the leaves, 
I jut has been reported to produce .small, 
long, dark streaks on the petioles, 

Stemphylium leaf spot, or target spot, 
is caused by Slemphylium sarcinijorme. It 
is common on red clover in the United 
States and Europe It is not considered 
a major disease of red clover, but its 
importance may have been underesti- 
mated. It can cause serious defoliation 
and losses of 15 to 40 percent of the 


223 

crop in individual fields. It is known to 
occur in nature only on red clover. 
L. J. Krakover, working at the Michi- 
gan Agricultural Experiment Station 
at East Lansing, inoculated sweetclo- 
ver, alsike, white, and crimson clovers, 
as well as alfalfa, vetch, and several 
other legumes, but he was not able to 
infect them. James G. Horsfall, at the 
New York Agricultural Experiment 
Station at Ithaca, however, reported 
infection from artificial inoculations on 
alsike and white clovers, sweetclovcr, 
and alfalfa. That difference in ability 
of the isolates to cau.se disease suggests 
that the fungus may have more than 
one race. 

Symptoms are limited almost exclu- 
sively to the leaflets. Minute, light- 
brown spots, similar to the early symp- 
toms of some of the other leaf spot 
diseases, appear first. Fully developed 
spots are mostly oval or round and 
about one-fifth inch across. I’hey may 
lie larger when only a few' occur on a 
leaf. The most characteristic symptom 
is the occurrence of concentric rings 
within th^ lesion, suggesting the name 
target spot. The center of a typical 
spot is dark and distinct. It is .sur- 
rounded by alternately light and dark 
rings. The darker rings are sepia to 
dark browm. The lighter ones are 
ocher to light brown. The darker 
rings near the center of the lesion arc 
narrow' and ridged. The color con- 
trast between the lWo outermost rings 
is very sharp. Spots are most abundant 
near the margin of a leaf but may 
occur anywhere. Frequently they coa- 
lesce, killing large areas of the leaf 
and causing defoliation. Symptoms on 
the stems and petioles are uncommon 
bat when they do occur they appear as 
dark-brown to black linear streaks. 

A related fungus, S. hotryosum, at- 
tacks alfalfa and sometimes occurs on 
red clover. 

Blackpatch, caused by an uniden- 
tified fungus, was first recognized in 
Kentucky as a disease of red and w'hite 
clovers. It has also been reported from 
Wisconsin, West Virginia, and Geor- 
gia. It has generally been considered 



224 YIAtiOOK OF AOtICUlTUtI 1»5I 


of little economic imj^rtancc but 
occasionally causes losses in local areas. 
In heavily infected fields of red clover 
the seed yield may be reduced at least 
50 percent. 

In addition to red and white clovers, 
the disease has been found on soybean 
{Glycine max), cowpea (Vigna sinensis), 
kudzu (Pueraria thunbergiana), and blue 
lupine (Lupinus angustifolius). It has not 
yet been reported as occurring in na- 
ture on alsike clover, crimson clover, 
alfalfa, and the sn^eetcJovers, but all 
of those crops have been infected by 
artificial inoculation. 

Biackpatch attacks the leaves, stems, 
flowers, and seeds. Under normal con- 
ditions it occurs in patches. Otherwise 
it appears only on scattered plants. 
Leaf lesions are similar in size and 
color to those caused by Stempkylium 
sarciniforme. They vary from brown to 
grayish black and usually have con- 
centric rings. Large areas of a leaf may 
be affected. Sometimes all the low^'er 
leaves are killed. Greatest damage 
* results from the girdling of the stems 
beneath the flower head or from direct 
infection of the flow^ers before the seeds 
arc fully developed. The fungus is 
seed- transmit ted. It also causes seed- 
ling blight. Examination of diseased 
plant parts with a hand lens usually 
reveals the presence of coar.se, dark, 
aerial mycelium, a characteristic that 
helps in diagnosing the disease. 

Treating the seed with a fungicide 
should aid in preventing initial infec- 
tion. In hayfields, losses can be reduced 
by early harvesting. Crop rotation and 
sanitation should also be helpful. 

Curvularia leaf spot is caused by 
Cufvularia trifolii. It sometimes causes 
considerable wilting and premature 
dying of leaves of Ladino clover in the 
eastern United States. It was first dis- 
covered in 1919, when it caused minor 
damage to white clover near Wash- 
ington, D. C. Since 1940 it has oc- 
curred more frequently, presumably 
because of the widespread use of 
Ladino clover along the Atlantic sea- 
board. Up to 25 percent of the leaves 
may be attacked and damaged. Ladino 


clover may be more susceptible than 
common white clover. It is possible to 
infect other species of clover in the 
laboratory, but the disease has not 
been found attacking them in the 
field. Infected leaves usually have a 
large yellowed area, which soon turns 
watery gray and translucent, then 
light brown. A yellowish band usually 
outlines the advancing edge of the 
infected part of a leaf. Diseased areas 
that originate at a leaf tip sometimes 
become V-shaped. Infected leaves wilt, 
then shrivel and die. Sometimes the 
dead V-shaped part of a leaf curls. 
The fungus can invade the entire 
leaflet and grow down the petiole, 
causing complete wilting of the leaf. 
Apparently it does not attack stolons. 
The disease develops most rapidly 
during w'arm, wet weather. A temper- 
ature of 75®“'8 o® F. is most favorable. 

Cercospora leaf and stem spot, also 
called summer black stem, is widely 
distributed in the United States and 
Europe. It is commonly found on 
most of the true clovers, including red, 
alsike, white, crimson, zigzag, hop, 
{Trifolium agrarium), and many others. 
A similar disease occurs on alfalfa, 
sw^eetclovcr, black medic, and related 
species. Cercospora zebrina is the most 
important pathogen on all of those 
hosts except sweetclover. C. davisii is 
the most important (or the only) 
species infecting sweetclover. Damage 
varies from year to year, depending on 
weather conditions, but the disease is 
always present and frequently causes 
excessive premature defoliation. 

Symptoms vary somewhat on the 
different host plants. Leaves, stems, 
petioles, petiolules, and seeds may l>e 
attacked. Leaf lesions are usually 
angular and more or less confined by 
the veins. The size and shape of the 
lesions vary from rather small, linear 
spots on red clover to large, almost 
circular ones on sweetclover. Appar- 
ently atmospheric conditions and the 
kind of tissue influence tlie size and 
shape of the lesions. Color of the spots 
also varies considerably. On the true 
clovers the general tone is reddish or 



THl MANY AliMINTS 0|( CLOVEI 


smoky brown. On swectclovcr it is 
ashy gray. When conditions favor 
sporulation of the pathogens, a silvery- 
gray down develoj^ on mature lesions. 
Lesions on the stems and petioles are 
somewhat sunken but with colors like 
those of the leaf spots. Stem infections 
are serious as they cause the distal 
parts to wilt and die. Seeds may also be 
infected. The disease may be dis- 
tributed on the seed. 

Infection may occur at any time 
during the growing season on plants of 
any age, but the disease is usually most 
abundant in late summer and autumn. 
On swectclovcr the disease is most 
conspicuous on second-year plants 
after they have started to bloom and is 
more severe on plants that have been 
cut or grazed. 

Little is known concerning the con- 
trol of this leaf and stem spot. Remov- 
ing old crop residues and crop rotation 
help reduce damage. Infected seed 
should be treated with a fungicide 
to reduce seedling infection. The 
development of resistant varieties of 
legumes seems to hold out the most 
hope for controlling it. 

Pseudoplea leaf spot, or pepper spot, 
is due to Pseudoplea trifolii, a fungus 
that attacks the true clovers and less 
commonly alfalfa. It occurs through- 
out the United States, Canada, Eu- 
rope, and Asia. The disease has not 
been reported on sweetclover. It is 
important on Ladino and white clover 
in Northeastern and Southern States, 
where severe infection frequently 
causes premature yellowing and defo- 
liation of lower leaves. It occurs 
throughout the growing season but 
develops most abundantly in cool, wet 
weather. 

Tiny, sunken black flecks develop 
on both surfaces of leaves and on 
petioles. The flecks rarely reach a 
diameter of more than a few milli- 
meters but frequently are very numer- 
ous — hence the name pepper spot. 
Later they turn gray with a dark, 
reddish-brown margin. Heavily in- 
fected leaves and petioles become 
yellow, wither, turn brown, and 


225 

collapse as a dead mass. Flower stalks 
Md floral parts may also become 
infected and be killed. 

Infection occurs from spores that 
develop on dead, overwintered leaves 
and petioles. Sp>ots can usually be 
found on the first hew leaves that 
emerge in the spring. 

No practical control measures are 
known, but plants differ in suscep- 
tibility, and breeding for resistance is 
possible. 

Bacterial leaf spot, caused by Pseu- 
domonas syringae^ does not usually 
cause serious damage, although it is 
widespread in the United States and 
has been reported in Italy and 
England. Wet w^eather favors its 
rapid spread and development. Hot, 
dry weather checks it. 

The disease may appear at any time 
during the growing season. It is most 
conspicuous on the leaflets. It also 
affects the stems, petioles, petiolules, 
stipules, and flower pedicels. First 
symptoms are tiny, translucent dots 
on the lower leaf surface. The spots 
enlarge and fill the angles between 
the veins. They are tiny and black 
except for the margins, which retain 
a water-soaked apjDcarance. In wet 
weather a milky-white bacterial ex- 
udate may develop as a thin film or 
as droplets. On drying, the exudate 
becomes a thin, incrusting film, which 
glistens in the light. Tissues surround- 
ing the spots arc yellowish green. 
Infection may be so abundant that 
whole sections of a leaflet are killed. 
Mature leaves are often perforated 
and frayed because of the drying and 
shattering of parts of the diseased 
tissues. Lesions on the petioles and 
stems are dark, elongated, and slightly 
sunken. 

Several clovers, including red, alsike, 
white, crimson, zigzag, and Berseem 
{Trijolium alexandrinum), are known 
hosts of this pathogen. Isolates from 
different areas differ in pathogenicity. 

Several other bacterial diseases have 
been found on clovers, but none of 
them is important in America. 

Sooty blotch, caused by Cymadothea 



226 


YEARBOOK OF AGRICULTURE 1953 


Iri/oiii, is one of the most conspicuous 
and easily identified leaf spots of 
clover. It is prevalent throughout 
North America, particularly in the 
southern part of the United States, 
and in Europe. It is most common in 
alsikc, red, and while clovers. Fre- 
quently it reaches epidemic propor- 
tions on crimson clover. It also has 
been reported as occurring on some 24 
other true clovers. 

In the Somhern States the di.sease 
appears in the spring. In Northern 
States it is more prevalent in late 
summer and fall. The earliest symp- 
tom is tlie development of minute, 
olive-green dots mostly on the lower 
leaf surface. The dots enlarge and 
become thicker and darker until they 
acquire the appearance of velvctv 
black, angular, elevated patches or 
warts. In the fall the warts arc re- 
placed by other black areas, which 
have a shiny .surface. C’hlorotic and 
later necrotic spots appear on the 
upper surface of the leaf iiniriediately 
al)Ove the warts. When .sjxHs are 
abundant the entire leaf may turn 
brown, die, and fall off. Sooty blotch 
is of considerable economic impor- 
tance on crimson clover, cau.sing re- 
duction in .seed yield. 

Powdery inildew', cau.sed by Ery- 
siphe polyQoni, is a common and wide- 
spread disease of clovers. It prol.»abiy 
occurs wherever liiey are growm It 
can cau.se reductions in the yield and 
quality of hay. Ordinarily it is of little 
consequeaice on tlie first hay crop Init 
is more abundant on the second. It 
can attack plants at any stage of ma- 
turity but develops best during the 
cool nights and warm days ol the latter 
half of summer and fall. Long spells of 
dry weather favor its development. 

The pathogen has been recorded on 
some 359 species belonging to 154 
genera. It consists of a number of 
physiologic races, which differ in their 
ability to attack different genera and 
species of hosts and different varieties 
of a species. European varieties of red 
clover have been reported generally 
to be more resistant than American 


varieties, bur American strains are 
now available that are highly re- 
sistant. Wisconsin Mildew Resistant is 
one of them. Most varieties contain a 
few resistant plants. 

At first barely perceptible patches of 
fine, white, cobwebby mycelium de- 
velop on the upper surface of the 
leaves. The patches enlarge and merge, 
the fungus sporulates, and the leaf 
surface appears as if it had been dusted 
with white flour. Symptoms may also 
occur on the lower surface of leaves. 
Severe attacks can make whole fields 
appear white. 

Rusts of glovers arc widely dis- 
trilnjted in the humid and subhumid 
areas of the wwld. Damage is difficult 
to assess l:)ecause heavy infection usu- 
ally docs not occur until late in the 
summer. Occasionally heavy infection 
occurs, and severe loss results when a 
grow'cr attempts to produce tw'o seed 
crops in a season and leaves the ac- 
cumulated rust-infected old growth to 
infect the new grow’th. 

'Fhree common varieties of ru.st 
attack Clovers. They cannot be dis- 
tinguished on the basis of symptoms 
but can be differentiated t.>y differ- 
ences in their capacity Lo infect the 
various clovers. For example, the 
variety of rust on alsike clover (i'ro~ 
mim trifolii hybridi) infects only alsikc, 
while the rust on red clover (U. tri^ 
Join fallens) infects red, zigzag, crimson, 
Berseern, and several other clovers. 
The ru.st on w'hite clover (f/. Irijolii 
Irifolii-rtpentu) does not infect red or 
zigzag clovers but does infect crimson 
and Berseern, besides white clover. 
These rust fungi differ from thi>sc 
causing the cereal rusts in that they 
can complete their entire life cycle on 
a clover species and do not require an 
alternate host. 

The most conspicuous symptom of 
clover rust is the uredial, or brown rust, 
stage, in which round or irregular, 
paJe-brown pustules, surrounded by 
the torn epidermis, appear on the 
lower surface of leaves and on the 
petioles and the stems. Sometimes in 



THE MANY AILMENTS OF CLOVER 


winter in the South and early spring in 
the North, small swollen whitish-to- 
yellow clusters of tiny cuplike struc- 
tures occur on the stems, petioles, and 
large veins of the leaves. These are 
called the accia and may cause dis- 
tortion of the affected leaves and peti- 
oles. Rust fungi in the tclial, or black 
spore, stage overwinter on the debris 
of diseased plants. 

Fungicides such as sulfur can be used 
to control the clover rusts, but rarely is 
it practical to use them. Resistant 
plants exist in present varieties and 
resistant strains can be developed if the 
importance of the disease warrants it. 

All common clovers are susceptible 
to several viruses, some of which are 
widely distributed. Most of the legume 
viruses have a wide range of hosts — so 
that clover viruses infect not only 
clovers hut also other legumes, and 
viruses of other legumes attack clovers. 
Some viruses from nonlegume hosts 
such as tobacco, gladiolus, potato, and 
some w^ecds can readily infect clovers. 
That means that many of the virus 
dise^es spread from one crop to an- 
other. The extent of the spread and 
amount of infection usually depend 
on the kinds Mid numbers of insects 
present. Aphids are probably the most 
important carriers. 

Symptoms \’ary with the virus and 
host. Most of the clover viruses are 
systemic — that is, they are present in 
all parts of the plant. The most con- 
spicuous symptoms are usually found in 
the leaves. They include vein c hlorosis, 
mild to severe mottling, chlorotic 
patches between the veins, and other 
abnormal combinations. Sometimes the 
leaves are curled, puckered, or ruffled. 
Some viruses cause a reduction in vigor 
as indicated by a general stunting of 
the plant. Others have no apparent 
effect on vigor. V^iruscs that have little 
efl'ect on one clover may kill another. 
Symptoms of most clover virus diseases 
are conspicuous during the cooler pe- 
riods of the growing season and some- 
times disappear temporarily or are 
masked during hot w^eather. Weaken- 


227 

ing caused by virus disease may predis- 
pose plants to attack by other patho- 
gens (especially those that cause root 
rots) or prevent them from surviving 
severe winters or prolonged droughts. 

The virus diseases reported to occur 
on the more important clovers are: 

Red clover: Red clover vein-mosaic, 
common pea mosaic, yellow Irean 
mosaic, potato yellow dwarf, American 
pea streak, New Zealand pea streak, 
Wisconsin pea streak, pea motile, pea 
wilt, alfalfa mosaic, sub clover mosaic, 
ring spot, broad bean common mosaic, 
broadbean mild mosaic, and cucum- 
ber mosaic. 

Alsike clover: Alsikc clover mosaic, red 
clover vein-mosaic, sub clover mosaic, 
common pea mosaic, pea mottle, pea 
wilt, and New Zealand pea streak. 

White clover: Alfalfa mosaic, red clover 
vein-mosaic, yellow bean mosaic, pea 
mottle, pea wilt, American pea streak, 
New' Zealand pea streak, and broad- 
bean mild mosaic. 

Crimson clover: Alfalfa mosaic, red 
clover vein-mosaic, sub clover mosaic, 
alsike clover mosaic, common pea 
mosaic, yellow bean mosaic, pea 
mottle, pea wait, American pea streak, 
potato yellow dw^arf, and broadl)ean 
common mosaic. 

Sub clover: Sub clover mosaic and 
yellow bean mosaic. 

Swectcloiers: Alfalfa mosaic, alsike 
clover mosaic, red clover vein-mosaic, 
sub clover mosaic, common pen mosaic 
yellow bean mosaic, pea mottle, pea 
wilt, American pea streak, tobacco 
streak, tobacco ring spot, and broad- 
bean mild mosaic. 

Little has been done to control the 
virus diseases of clovers. Some of the 
newer insecticides kill the insect vec- 
tors. When possible, clover'' should not 
be grown close to other legumes, 
particularly peas or beans. The ulti- 
mate solution is to develop varieties 
of clovers resistant to the most prevalent 
and injurious virus diseases. That 
remains to be done. 

Several pathogens can attack the 
floral parts of the clovers under some 



228 


rEARBOOK OF AORICUITURE 1959 


conditions. They are of importance 
only when they interfere with seed 
production. 

Anther mold {Botrytis anthophila) was 
first reported from Russia in 1914 and 
has since been found to be widely dis- 
tributed in Europe. In the United 
States it has been found to a limited 
extent in Oregon. It has no apparent 
efiect on the plants until flowering 
time; when it destroys the normally 
yellow pollen grains, replacing them 
with the gray spores of the fungus. If 
abundant it can reduce seed setting. 

Earle W. Hanson, a native of 
M innesota^ joined the Department of Agri^ 
culture in ig37- From igjy to 1346 he was 
employed by the division of cereal crops and 
diseases of the Bureau of Plant Industry^ 
Soils ^ arid Agricultural Engineering doing 
research at the Minnesota Agricultural 
Experiment Station on the diseases of hard 
red spring wheats and the development of 
disease-resistant varieties of wheat. Since 
1^46 he has been employed jointly by the 
division of forage crops and diseases of the 
Bureau and the University of Wisconsin, 

Kermit W. Kreitlow is also a mem- 
ber of the division of forage crops and dis- 
eases and is stationed at Beltsville^ Md, He 
is a graduate of the University of M innesota 
and I^uisiana State University. Dr, Kreit- 
low has been engaged in work on forage 
crop diseases since 1341 . 



Crown wart of alfalfa. 


Sources of 

Healthier 

Alfalfa 

Fred R. Jones., Oliver F. Smith 

Alfalfa as a forage crop in the United 
States is recognized as consisting of 
strains of Medicago sativa and of hybrids 
of that species with Medicago falcata. M, 
sativa occupies the southern and central 
alfalfa regions. The apparent hybrids 
appear to use the superior hardiness of 
the M. falcata parent to give them lon- 
gevity in the severe climate of the 
northern part of the range of the crop. 
Thus two species, which vary greatly 
within themselves, produce in their 
combined resources a crop that ift the 
hands of progressive agriculturists has 
spread across a wide range of climate. 

As often happens in such wide and 
intensive culture, serious diseases have 
appeared. Many have reduced the 
quantity and quality of alfalfa forage. 
To overcome the loss, breeding for re- 
sistance has been undertaken. Already 
those efforts have demonstrated that 
the qualities of two variable species can 
be utilized to develop resistance to 
many of the diseases. 

The evaluation of the resistant char- 
acters is one of the major tasks that 
face pathologists and breeders. There- 
fore in this discussion of diseases of al- 
falfa, we emphasize sources of resist- 
ance. 

Of the NONPARAsme DISEASES, win- 
ter injuiy is often the cause of weak 
growth in the spring and the subse- 
quent unthrifty condition of many 
plants. Besides, injured tissues often 
become the avenues of entrance for 



SOURCIS OF HEALTHI6R ALFALFA 


parasitic micro-organisms that weaken 
and kill the plants. In the northern al- 
falfa regions, effects of winter injury 
can be modified by using hardy or 
adapted varieties. Also, if snow is ex- 
pected to be a factor in winter protec- 
tion, fall growth should be left to hold 
the snow. If a field is damaged, but is 
to be saved, one should avoid early 
cutting or grazing the following spring. 

Occasionally some white spots occur 
around the margins of upper leaves on 
scattered plants. Sometimes the spot- 
ting seems to be an inherited character, 
which develops in plants several years 
old when the crown begins to decay. 
Sometimes it develops after winter in- 
jury. It has been produced experimen- 
tally in young plants grown in soil 
deficient in potash. Thus at times its 
occurrence is taken as an indication of 
potash deficiency. White spot also de- 
velops extensively in response to mois- 
ture change in irrigation, although the 
spotting then may develop over the en- 
tire leaf instead of only at the margins. 

Yellowing and dwarfing of alfalfa is 
often widespread from feeding of the 
potato leafhopper. Yellowing may also 
be an indication of boron deficiency. 

Of the virus diseases, alfalfa dw'arf 
comes first. It i.s known to occur only in 
California. It was first reported in 1931 
when it was localized in Riverside and 
adjacent counties south of the Teha- 
chapi Mountains. In 1952 the disease 
occurred over much of the San Joaquin 
Valley and caused a rapid thinning of 
second- and third -year stands in about 
a third of the alfalfa acreage of Cali- 
fornia. 

Alfalfa dwarl is due to a virus that 
leafhoppers and spittlcbugs^arrv from 
plant to plant. Fourteen species of 
leafhoppers and four species of spittle- 
bugs can carry it. Two species of leaf- 
hoppers {Craeculacephala minerva and 
Corneocephala Julgida) are the most im- 
portant vectors in the field. When the 
leafhoppers feed upon dwarf-diseased 
plants, they take the virus into their 
bodies and retain it for several months, 
during which time they may spread 


229 

the virus to healthy plants when they 
feed. Only leafhoppers can spread the 
disease from field to field. 

The virus that causes alfalfa dwarf is 
the same virus which causes Pierce’s 
disease of grapevines. 

Alfalfa plants infected with the dwarf 
virus gradually lose vigor for several 
months. Stems are short and spindly. 
The leaves get smaller and often seem 
darker in color than leaves of healthy 
plants. Internally, gum forms in the 
water-conducting elements and the 
woody portion of the roots and crown 
becomes yellow or brown. Susceptible 
plants usually die 6 to 8 months after 
infection.’ 

Tests conducted at the University of 
California indicate that no available 
variety of alfalfa carries any degree of 
resistance greater than that shown by 
California Common. Many varieties 
are much more susceptible. In fields of 
California Common, four or more 
years old, practically all plants were 
dead or infected with the virus; about 
one plant in each 2,000 square feet 
made a normal growth even though 
root symptoms showed that the plants 
had been diseased i or 2 years. Several 
such plants were selected and self- 
pollinated to produce seed. Progenies 
from most of the selected plants were 
quite tolerant to the virus and main- 
tained normal growth tw'o to three 
times as long as unsclected California 
Common. The plants were used to 
develop a variety — California Com- 
mon 49 — highly tolerant to the dwarf 
virus. 

WiTCHES’-BROOM OF ALFALFA WaS 

first recognized in the United States 
in 1925. It also occurs in Canada and 
Australia. Except in a few localities 
where outbreaks were severe, it has 
been considered of minor importance. 

The first outbrezdc occurred in Salt 
Lake County, Utah, in 1925. In some 
fields 60 to 65 percent of the plants 
were infected. Farmers used short crop 
rotations to combat the disease effec- 
tively. 

An outbreak occurred in Methow 



YEARBOOK OF AGRICULTURE 1953 


230 

Valley and Yakima Valley of Wash- 
ington in the 1930’s. When first re- 
ported, infection ranged from 25 to 60 
percent. Later surveys showed that in- 
fection reached 80 percent. A similar 
outbreak and build-up of the disease 
occurred in the Nicola Valley, British 
Columbia, in 1943. 

Some of the alfalfa seed fields in the 
Uintah Basin, Utah, have shown a 
marked increase in the occurrence of 
witches’-broom since 1950. In some 
fields 15 to 20 percent of the plants 
have been infected. In 1943 witches’- 
broom could .scarcely be found on the 
Yuma Mesa in Arizona; 8 years later 
some of the fields had qo to 30 percent 
infection. 

Witches’-broom slowdy modifies the 
appearance of affected plants in .sev- 
eral ways. Plants that show symptoms 
for the first time have many more 
stems, an erect habit of grow'th, and 
slight marginal chlorosis of the younger 
leaves. In the advanced stage of in- 
fection the plants arc sev
Internet Archive Audio

Featured

Top

Images

Featured

Top

Software

Featured

Top

Books

Featured

Top

Video

Featured

Top

Mobile Apps

Browser Extensions

Archive-It Subscription

Save Page Now

Full text of "Plant Diseases"

See other formats
